The first step is to learn what drives actually exist. There are three possibilities: the extra drives are not there; they are there, not defective, but not connected; or they are there but defective. You will need to look inside the case. I'm just assuming that this is a desktop computer; consumer laptops don't have places to hold more than one drive internally.
WARNINGS:
If you aren't tech-savvy, you are probably sure that anything you touch inside the computer will ruin something. You won't feel confident unless the risk is recognized and addressed. I've included this section just to make you feel more comfortable. The biggest risk is from your being so focused on what you are doing with one hand that you forget you are waving a screwdriver around in the other. Stay aware of where both hands are and you will do fine. I'll explain everything you need to know. You won't be doing anything that can destroy your computer, electrocute you, or burn your house down.
For the initial look, power down the computer and then disconnect the power cord. If you find that there are extra drives in the machine, it might be necessary to turn the computer on with the case open, but we can deal with that later, if necessary. You can operate the computer temporarily with the case open. Just don't go poking around inside when it's on if you don't know what you're doing.
Opening the case
Almost all computer cases open one of two ways. The vast majority of cases currently in use open by sliding off a side panel. In some early cases, the sides and top were one "U" shaped metal shell. Look at what type yours is.
One piece shell: If the sides and top are a continuous metal shell, it will be held on in one of two ways. Some had a couple of screws on each side near the bottom. After you remove the screws, the shell will pull straight up. Others had a lip that wraps around the back with a few screws in it. On those, after removing the screws, move the shell slightly toward the back to separate it from the plastic front of the case before you can pull it up.
Side panel: If it is a side-opening case, the side you want is almost always the one on the left as you look at the front of the computer. It is secured with two or three screws in back through a lip on the panel. Sometimes these are "thumb" screws to make it easy; they will have large heads with a molded pattern that you can easily grab and turn with your fingers. If not, they will be normal screws that use a regular screwdriver; no special tools required. Once you remove the screws, the panel comes off either by sliding it all the way back, or sliding it back an inch or two to clear some tabs, then taking it off. It is designed to be snug when it is fully closed, so it may need a little nudge to start to move it (you won't have to force it with tools, just expect that it might take a little hand pressure). Sometimes the fastening lip in back has a little molded finger grip to give you something to grab to nudge it off. Otherwise, just put your hand flat against the side, press enough so you have some friction between your hand and the side, and push toward the back.
For either style, once you open the case, touch somewhere on the metal chassis of the case for a second. This will make sure you don't zap something inside with static electricity. If you get up and walk around, touch the chassis again before you touch anything else inside.
Closing the case
With either type of case, just reverse the procedure to put it back together. With the side panels, you have to get the panel seated back in a track at the top and bottom. If it doesn't slide closed easily, it means you don't have it seated. Just play with it. Before you slide it, it should look like it is sitting flush at both the top and bottom. Align it so it looks flush and then put a little hand pressure at the top and bottom to keep it aligned until you get it engaged in the sliding track.
Getting the case open and closed is the "hardest" part of what you will be doing. The rest is just looking, unless you find a drive that the refurbisher forget to reconnect and then you will push on a few connectors.
Where to look
Whatever hard drives are in your computer will be at the front. The internal metal "chassis" of the case includes a metal cage that all kinds of drives mount in. Many computers have a limited number of openings in the front of the computer, and these are used for drives where external access is important. For example, CD/DVD drives need to eject their trays. The primary hard disk is often in one of these bays. Sometimes, an external opening is used for a panel of connectors of various kinds, maybe extra USB ports, slots for flash memory cards, etc. If that is the case, there will not also be any kind of drive in the bay behind it.
For expansion purposes, the cage often includes additional bays for drives that don't need external access. There will usually be mounting locations that orient the drive similar to the exposed ones, so their connectors are toward the back. In some cases, there is an additional mounting cage turned sideways so those drives' connectors face the side of the case that opens.
What you are looking for
Basically, any rectangular device mounted in the drive cage that is not a CD/DVD drive will probably be a hard disk. Drives will not be just anywhere inside the cage. There are specific mounting locations. They are roughly two inches apart and correspond to slots or holes for fastening the drives. Look at the location of the CD/DVD drive. Any other drives will be at roughly 2" intervals corresponding with the mounting slots.
If a drive is connected, it will be easy to spot. If you can't tell if a disconnected drive is hiding in a bay, use a flashlight to look as far into the back of the mounting cage as you can. If a drive is in there, you should see its connectors. Also, you can look for mounting screws. Some cases provide plastic rails that mount to the drives for easy installing/uninstalling with a release button. If your case doesn't have that, the drives will typically be secured to the cage with a couple of screws that fasten through the cage into the sides of the drive.
What you find
At a minimum, you should find your CD/DVD drive and your primary hard disk. If those are the only drives, you are done with the inside of your computer and can close it up again.
If you do find any other drives, see if they are completely connected (no exposed pins). There won't necessarily be four other drives in there; it could be one or two.
If there are other drives, there may be more things for you to do inside the case. You can either close it up and re-open it later or leave it open, reconnect the power cord, and turn it on again. Just be careful to keep stuff out of the open case, including your cat.
Report back what you find and we'll proceed from there.
