The first step to installing a slave disk is to buy the hard drive. Before you do this, make sure your computer has enough space and hookups for it. Start by unscrewing the sides of the computer and removing the side panels. Check to make sure that you have enough space in there for a second hard drive. The hard drive is normally located towards the front of the computer, screwed between two panels, somewhere below the cd-rom drive. Most computers have plenty of space here to add more hard drives, but some smaller computers do not.
Once you have made sure you have enough room, check to see what hookups you have available on your motherboard. The easiest way to find where these hookups are is to follow the cord from the back of your current drive down to where it is connected to the motherboard. If this is a ribbon it is a IDE connector. SATA connectors are normally solid black cords with red connectors. If you bought your computer within the last five years, you are most likely using SATA. Even if your not, you may have SATA connectors on your computer.

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SATA ports look like the picture on the left. IDE ports look like those on the right. You need to make sure you have at least one of these available to plug in your hard drive. If you have a SATA drive, I strongly

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recommend you buy a hard drive with a SATA connection. When you go to the computer store or buy online check to make sure it is a SATA connection. Most will be nowadays.
Also, keep in mind that you will need to have connectors for these as well. Some hard drives come with them but just to be sure I would pick up a corresponding cord (SATA or IDE) with your hard drive. Also, you'll need to make sure you have a power connector. These are the cords coming out of the powersupply. These are either large white connectors (Molex) or small black ones (SATA power)-Refer to this power supply connector guide to see which ones you have. Make sure you have a free one that can reach the place you plan to install the hard drive--if you are getting an IDE hard drive go ahead and make sure you have a Molex power connector. If you have a SATA hard drive it'd be a safer bet to make sure which one it uses, though they almost all use SATA plugs. If you do not have free connectors of the correct type, you can pick up an adapter for only couple dollars at the computer store.