I bought my Sister's old tower. This is with a Gateway DX4380 desktop computer.
I like to add a second harddrive to all my systems, my main Asus one has three. I decided to use one of my backup hard drives in this sytem. I took off the side panel only to discover nowhere to mount a second drive. There was the DVD drive in the top, a second enclosed compartment with wires sticking out the back, under that. I did not know what this was for at first. The hard drive was attached under that. No place for a second.
I was curious what that second compartment was for, so I went to the front of the computer and lowered the flap or door that hid whatever was there. And I discovered it was a caddy for an hard drive.
As in my picture the little sign next to the red handle says, "Please shut down the system before swapping the hard drive." Repeated under in french. When I opened it there was no drive inside and was curious why they didn't use it. I assumed it was broken. When I put in my hard drive and plugged in my computer, powered it on, sure enough it didn't pick up the drive at all. Just for the record, I know the drive is good.
I checked Disk management by right-clicking on the start button and clicking on it in the menu. Sometimes Windows doesn't assign a drive letter and that's why it seems unavailable. But still nothing.
Read more about assigning drive letters here: Fix Windows Doesn't Automatically Assign A Drive Letter
I disected my computer again to see if the cables attached to this drive caddy could be replaced. Then I noticed the power cable wasn't hooked up. So I plugged it in to the approriate connector, hooked up my system again, put in an hard drive and it worked.
So the moral of this story, ... check your cables. That's all I got. :-).