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All standard IDE controllers are capable of connecting one or two hard drives (SCSI controllers can handle seven, but most people aren't that lucky). But there are three basic things to be wary of when deciding to upgrade.
If you are going to have two hard drives, is there room inside your computer case for another drive? Desktop systems may already be full; tower cases usually have room for expansion. Also, will your computer's power supply support an additional drive? If there's a spare 'Px' power connector lying inside, then there's no problem. If not, you can always buy a Y-splitter to branch off an existing power connector, but first make sure the power supply can handle the extra wattage.
Second, if you plan on installing a drive larger than 528MB, will your computer support it? All new '486 and Pentium computers will handle large hard drives, but '386 and older '486 computers have an upper limit of 1024 cylinders (if you try to use a larger number, the BIOS will simply chop off 1024. In other words, your 540MB hard drive just became 12MB!) If you're not sure about your particular computer, check with the manufacturer of the motherboard or BIOS. Even if your particular BIOS doesn't support the larger hard drives, some hard drive manufacturers will provide a software utility that will override the BIOS in order to give you access to the entire drive.
The last and most important question is, what is the make and model of your existing hard drive and the one you want to add? When you have two hard drives connected together, one of the drives must be set as the 'master', and the other as the 'slave'. Although most contemporary drives work well together, be aware that some models are not quite compatible with others, and some require different jumper settings depending on what model the companion drive is. My best advice is to buy a well-known brand and try it out. If it doesn't work out as a slave drive, try it as the master (and vice-versa). Jumper settings for each drive can be found in the user's manual for the drive; if you are missing the manual for a drive, contact the manufacturer.
Once you have physically installed the new hard drive, there are a few more steps you need to take to prepare it for use. In your computer's setup utility, make sure it has the correct settings for your hard drive. These settings will be listed in the documentation that came with the drive, and in some cases may be on a sticker on the drive itself. Newer computers have an auto-detect feature which greatly simplifies the setup; just tell it to auto-detect the drive, and the computer will fill in the rest of the numbers for you.
Next, you need to create a partition on the drive so that it will be recognized by DOS. If your drive requires special disk management software, follow the directions provided by the drive's manufacturer. Note that if you have a really large drive, you may discover the maximum partition size. For DOS and Windows 95 partitions that use FAT-16, you cannot create a single partition larger than 2GB. Therefore if your hard drive is larger than this, you will need to create an extended partition to handle the remainder of the drive. See the notes on how DOS assigns drive letters. The next paragraph applies if you have a small hard drive (less than 528MB) or your computer will handle larger hard drives on its own.
If the new drive is your primary drive, you will need a DOS Setup disk (or Windows 95 Startup disk) to start the computer. After starting the computer and bringing it to the DOS prompt, you can use the command FDISK to create a Primary DOS partition. In most cases, you will want the partition to occupy the entire hard drive. After creating the partition, you will need to reboot the computer again. Then use the FORMAT command to prepare the partition for use as follows: if this is your primary drive, type "FORMAT C: /S" (the /S option allows the computer to start directly from the hard drive.) If this is a secondary drive, type "FORMAT D:".
After the drive has been formatted, you may proceed to install DOS or Windows 95 if this is your primary hard drive, followed by whatever other software or files you wish.