Dell Dimension Hard Disk Upgrade Notes

compiled by Robert Hancock

This page has some miscellaneous information on upgrading hard disks in Dell machines.

ATA Interface and Transfer Speeds:

ATA is the most common hard disk interface (the other major one is SCSI, which is much less common and won't be covered here). It's also commonly referred to as IDE, but technically that isn't the proper name for it, so I'll call it ATA. (ATA actually stands for AT Attachment, so named because it was originally used on AT-class, 286 processor machines.)

Most systems have 2 ATA channels available on the motherboard, each on their own cable. The first (primary) channel is usually used for hard drives, while the second is usually used for CD-ROMs, DVD drives, CD-RWs, Zip drives, etc. Each channel can have up to 2 drives on it. The first drive is called the master, and the second (if any) is called the slave.

Usually, drives can be set to be either master or slave by changing jumper settings on the drive. Dell systems often use Cable Select, which allows the drive to set its master/slave setting based on which connection on the cable it is connected to: the one on the end of the cable is master, while the one on the middle connection is the slave. For this scheme to work, the drives' jumpers have to be set to use Cable Select. You can always set the master/slave settings on the drives in the usual way and override the Cable Select.

Note that if you have only one drive on a channel, it should always be set to master, not slave. Also, you should never have both drives on a channel set to master or both set to slave. As well, if only one drive is on a channel, always connect it to the connector at the end of the cable, not the middle one. If you use the middle one, the unterminated end of the cable can cause interference and result in the interface not working properly.

Most relatively recent systems (i.e. Pentium II/Celeron or later) use a set of ATA transfer modes called Ultra DMA. There are three major Ultra DMA speeds: ATA-33, ATA-66, and ATA-100. These are also known as Ultra DMA modes 2, 4 and 5, respectively, and allow maximum data transfer rates over the interface of 33 MB/sec, 66 MB/sec and 100 MB/sec respectively.

Note that the speed of data transfers over the interface is seperate from the speed at which the drive can either read or write. Usually the drive's actual read/write speed is quite a bit slower than the interface transfer rate. However, the faster interface transfer rate can help when the requested data is already in the drive's onboard memory cache and doesn't have to actually be read in.

DMA refers to Direct Memory Access, which means that the drive (and controller) will copy data in and out of memory itself, rather than needing the processor to do the transfer to/from the drive's buffer (the older PIO, or Programmed I/O mode). In addition to using more CPU time, PIO is limited to 16 MB/sec. Therefore, you should always use DMA mode. In Windows Device Manager, if you view the properties for an ATA device, there should be a DMA checkbox under Settings. It should be checked. (If you are using a PCI ATA card, described below, they usually have DMA always enabled, so this checkbox may not be there.)

ATA-33 was the first major Ultra DMA mode to be used. It is supported by the onboard controller on all LX chipset motherboards (like on the Dimension D-series machines) and BX chipset motherboards (like the R, T and V-series machines), which use a chip called the PIIX4 to run the IDE interface, among other things. If you connect a drive supporting ATA-66 or ATA-100 to these boards, it will work, but the interface will run at ATA-33 speed.

The next major mode was ATA-66. It is supported, as well as ATA-33, on the L-series machines with the original 810 chipset (see the L-series page to see which ones these are), as well as on the B-series machines with the 820 chipset. If you connect a drive supporting ATA-100 to these boards, it will work, but the interface will run at ATA-66 speed.

The most recent mode is ATA-100. It is supported, as well as ATA-33 and ATA-66, on machines using the 815E chipset, like the Dimension 4100 machines, as well as on machines using the 850 chipset, like the Dimension 8100 machines, and on L-series machines that use the newer 810E chipset.

ATA-66 and ATA-100 require a different cable than was used with ATA-33 and older standards. It still has the same 40-pin plugs on it, but it actually has 80 wires in the cable - each signal wire is paired with a ground wire. This was needed to prevent interference when the interface runs faster than ATA-33. ATA interface chips that support ATA-66 and ATA-100 are designed to detect the use of an older cable and prevent the interface from running faster than ATA-33, so if you want the full transfer rate, you have to use an 80-wire cable. These cables are also compatible with older drives and motherboard interfaces, so there is no reason to buy an older 40-wire cable.

If your motherboard doesn't support the maximum transfer rate of a drive that you want to install, you can get the full transfer rate by installing an add-in PCI ATA-100 card, like those available from Promise and others, and connecting the drive to it. (Maxtor drive kits sometimes come with such a card as well.) This card will give you two more ATA channels which will support ATA-100 speeds. If you install such a card, you can still use the interface on your motherboard, so you can install up to 8 ATA devices in total. (You may want to connect devices such as CD-ROMs, CD-RWs, etc. to the motherboard interface, as these usually don't support over ATA-33 anyway.)

Note that only one device on an ATA channel can be active at once, so only one device can be reading or writing at the same time. For this reason, if possible it's a good idea to try to spread the devices over as many channels as possible, so devices aren't waiting for others to finish before they can start working. (Obviously this is much easier if you have 4 channels to work with, as you do if you install an add-in PCI ATA card.)

So, how much difference does the ATA interface transfer speed make, anyway? Not as much as some people think. Just because a drive supports ATA-100, for example, doesn't mean it can transfer to/from the disk at anywhere close to 100 MB/sec. The only real benefit to speeding up the interface occurs if the current interface speed is slower than the maximum sustained transfer rate of the drive (how fast it can actually read and write), because in that case the drive will be waiting on the interface and slowing things down. Almost all new drives support ATA-100, but currently the fastest drives only transfer a bit faster than ATA-33 speeds (around 40 MB/sec, for example), and none approach maxing out an ATA-66 interface. So if you install a fast new drive on an ATA-33 interface, you will likely see some benefit from using it on a faster controller, but if you already have an ATA-66 interface, there is little benefit to be had.

Drive Spindle Speeds:

The spindle speed of a drive refers to how fast the platters inside that store data rotate. Currently most drives are either 5400 RPM or 7200 RPM. Since 7200 RPM drives' platters rotate faster, data can be read to/from them at a faster rate, providing better performance. It also takes less time for a desired sector on a given platter to rotate into position under the head after the head moves to the track the sector is on, which allows reduced seek latency.

In my opinion, the only reason to buy a 5400 RPM drive is if you can't afford a 7200 RPM drive, or if you want a drive that's as big as possible (sometimes the biggest capacity drives aren't available as 7200 RPM models). Earlier 7200 RPM drives drew a lot of power, were noisier and ran hotter than 5400 RPM drives, but with today's drives, none of these is nearly as much of a problem.

However, any 7200 RPM drive isn't necessarily faster than any 5400 RPM drive, as RPM isn't the only factor that affects performance. Newer, higher-capacity drives with data more tightly packed on the platters will have faster transfer rates at the same spindle speed than older drives, for example.

Drive Installation Process:

Most new drives come with software (often a manufacturer-customized version of Ontrack Disk Manager) that makes the installation process very simple. Disk Manager will give you customized instructions on how to install and set up a new drive, as well as copying all your files onto the new drive for you (if you want to use the new drive as the boot drive, and you likely will since it will be faster).

Drives can come in either retail or OEM packages just like many other computer components. Retail packages usually include such things as drive mounting screws, instruction manual, sometimes a new cable, utility disks, etc. whereas OEM packages are often just the bare drive in an anti-static bag. For the latter, you can generally download all the installation software and manuals from the manufacturer's web site, though, if the drive didn't come with it. OEM packages are often cheaper.

Most Dell system cases have 2 mounting locations for hard drives - the first one being a vertical mount behind the bezel, and the second being a horizontal mount below the floppy drive. A notable exception to this is the L-series machines, which have only one location meant for a hard drive. If you have an L-series, and one of your external 3 1/2" bays is open, though, you may be able to put a second drive there and just leave the front cover for the slot on. The Dell manuals have good instructions for how to install/remove drives from the bays, so you should read them. If you don't have manuals, look on for documentation for your system.

One final note: If you install a second hard drive when you previously had only one, then your CD/DVD drive(s) will move up one drive letter. For example, a system with one hard drive and one CD-ROM would previously have the hard drive as C: and the CD-ROM as D: After installing the new hard drive, the two drives will be C: and D: and the CD-ROM will be E: Some CD/DVD based applications/games expect to find the disc on a given drive letter, and will fail to run if they can't find it. To fix this, you can reinstall the application/game, or fix all the drive letter references to point to the new drive. Disk Manager has the capability of doing this for you, and will probably volunteer to do so during the install process. PC Magazine also has a free utility called COA2 which can update all references to a particular path to something else. In this example, you would tell it to change all references to D:\ to E:\ .

This Microsoft article describes how DOS and Windows assign drive letters, and this one describes how you can move your CD's drive letters up pre-emptively before installing applications in order to prevent them from changing when you install a new drive.

Drive Capacity Limitations:

Some machines have limits on the size of hard drives that they can use. If you try to use a drive greater than these limits, it may only recognize part of the drive, not recognize the drive at all, or do other things like lock up when you try to access the drive.

Most relatively recent (Pentium II/Celeron and later) machines should not have any drive capacity limitations. The only limit that these machines might have is a hard drive size limit of 8.4GB on some BIOS versions of the H and D-series machines. However, these limits can be resolved by updating your system BIOS. For the H-series, you need BIOS A03 to use a drive over 8.4GB, whereas for the D-series you need BIOS A05 or later. Some machines have a 32GB size limit, but I am not aware of any Dell machines that have this limit (apparently this only affects machines with an Award BIOS, and all recent Dell Dimension machines use a Phoenix BIOS).

Older systems may also have hard disk size limitations, which may or may not be resolvable by updating the BIOS (sometimes no BIOS update is available to fix the limitation). As well as the 8.4GB limit, another common size limit with very old (486-era) machines is the 528MB limit.

If your system has a BIOS update available to fix a drive size limitation, that is always the best choice. However, if you install a drive using software like Disk Manager, it will detect if your machine doesn't normally support a drive of the size you are installing, and will use special drive overlay software to allow your machine to recognize it. This is sort of a "hack", though - for example, if you use drive overlay software, and you want to boot from a floppy disk, you must hit space bar right after your machine starts to boot, and then insert the disk. Otherwise, after the floppy boots up, you won't be able to access your hard drive.

In addition, any drives connected to an add-in PCI ATA controller card will be operated by the BIOS on the controller card, not the system BIOS. Therefore, any drives connected to such a card will not be subject to any hard disk size limitations.

One further note: Windows 95 (all sub-versions) does not support drives over 32GB, see this knowledge base article. The solution to this is to upgrade to Windows 98, Me or 2000. As well, on Windows 98 and 98 Second Edition, you must install this update to use drives over 32GB without problems. Finally, on Windows 98 and 98 Second Edition, you must install this update to be able to partition and format a drive over 64GB properly.

RAID Arrays:

RAID stands for Redundant Array of Inexpensive Disks. It allows you to effectively combine multiple drives into one, big virtual drive, or "array". There are several different ways of doing this. Some of the different RAID methods are:

RAID 0: The data is simply split across the disks, with no redundancy. This offers better performance as well as bigger capacity (the sum of all drive sizes) than a single drive, since more than one drive can be operating at once. However, if one drive fails, you effectively lose all the data on all drives (well, the data on the other drives is technically still there, but it would be very hard to recover it). You need at least 2 drives for this method.

RAID 1: Multiple drives are maintained with the same data on all of them. This offers a bit better performance, but no more capacity than a single drive. The big advantage is that if one drive fails, no data is lost, and you can even continue to use the array until the failed drive is replaced. With some controllers, you can even replace the drive without shutting down the machine. You need at least 2 drives for ths method.

RAID 5: Multiple drives are maintained with different sets of data on each, but any given piece of data is duplicated on at least 2 drives, so that no data is lost if one drive fails, and you can continue to access the array until it is replaced. It offers the performance benefits of RAID 0 as well as the redundancy of RAID 1, but usually needs more drives than those modes (at least 3 drives).

There is also RAID 0+1, which apparently uses 4 drives split into two pairs, each pair having the same data on them, but with the data split across pairs, giving a capacity of twice the size of each single drive. It offers most of the benefits of RAID 5, but is probably cheaper.

Traditionally, RAID has usually been used only in professional server machines, but some computer enthusiasts have started to use RAID arrays as well. In the past, RAID was only available with SCSI drives, which are usually more expensive. However, there are now some PCI ATA cards which support RAID, the best known ones being from Promise, though some other companies such as Iwill have them too.

These cards effectively create the illusion of the array being one big ATA drive to the operating system. The OS does not see the individual drives at all. After you set up the array hardware, you partition and format the array just like a regular drive. Note that if your array has an effective size of over 64GB, and you are using Windows 98, you must install the update mentioned above in "Drive Capacity Limitations" to be able to partition and format it properly.

I'm not a big expert on RAID, so if you need more info on this, some users in the DellTalk Upgrade-Hardware forum have experience with RAID and may be able to help you. Also, see the further reading section below.

Further Reading:

Major hard disk manufacturers: Maxtor, Seagate, IBM, Fujitsu.

The PC Guide has tons of information on hard drives, as well as many other computer topics. One of my favorite reference sites.

Promise Technology makes PCI ATA cards, some of which support RAID.

Iwill (makers of the famous Slocket II) produce a PCI ATA RAID card called the SIDE-RAID100.

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