Standard 3½" floppy drives are the same size as standard HDD units, even down to the location of the threaded mounting holes, so they fit. Now, if you are trying to mount a 3½" HDD into a wider 5¼" slot normally used for optical drives these days (but originally used for an older floppy design), that's another story. You used to be able to get a set of adapter rails to mount a 3½" device like a floppy drive into a 5¼" slot. In the case of doing this for a HDD, it's even easier because you don't have to adapt a front cover plate to allow access to the slot of a floppy drive. Just leave the original front cover in place, because you don't need access to a HDD. The adapter rails I used before looked like a pair of simple U-shaped pieces of channel with holes in the right places. You could screw them onto the sides of your 3½" HDD, and then slide the whole assembly into the 5¼" slot and put the right screws into the adapter's holes to fasten it in.