Well I've got an old SFF Dell Optiplex GX110 I've been using as a file server for a while now, and the old 40 gig HD it has was starting to get pretty full. So I decided to crack the thing open and cram an additional hard drive inside. I had another 40 gig drive lying around, and because I didn't feel like spending money on this thing it would do. Note the Pentium !!! and "Designed for Windows 2000/NT/98" stickers. 850mhz P3 is plenty for what this does.
Unfortunately there was nowhere for it to fit. I couldn't jam it under the existing hard drive because there wasn't enough room between the case, PSU, and motherboard. I kind of wanted to keep the optical drive in there in case it's needed again. So then MacGyver's floating disembodied head appeared and said
He must have possessed the computer. Crap started flying around everywhere! The drive tray climbed out. Cables ripped themselves out and made a pile at the other end of the room. When the dust settled, the new drive was sitting on top of the old one, where the floppy drive used to be, connected to power and IDE with a new 80 lead ribbon cable that came from somewhere. But the computer wouldn't boot! Turns out it can only boot off a hard drive if it's the master drive on the primary IDE connector. Another tornado of parts started. A lighter, pliers, a rock, and some electrical tape floated in through the window. Eventually, he found a combination that worked: New drive as slave mounted where the old one was, and the original one mounted in the (now slightly mangled) floppy drive mounts.
And as quick as he appeared, he was gone. Thank you, MacGyver.
The cover fits just fine and there's plenty of space around the new drive for air circulation:
And in case you ever need a slim CD-ROM drive for something, the ones that come in these computers use standard IDE and floppy power connectors
EDIT: I also made sure to mount it to a directory under the existing files partition, so it will seamlessly integrate with the Filezilla FTP server configuration. Yes, you can do that with Windows. So I tried to make this post more interesting than "I stuffed a second hard drive into a small PC's floppy drive area".