Internal hard drives/floppy drives (which go in the small 3.5“ bays) and internal
CD/DVD drives (which go in the large 5.25” bays) take up space in the case, so
make sure you consider how many drives you will need and what size slot they
require. Note that optical drives and floppy drives will need what are called
“external” slots (meaning they have a hole in the case through which their face
can be accessed, while hard drives do not need to be manipulated manually
often, so they can usually go into an “internal” slot. This is not to be confused
with an external drive, which doesn’t go into the case at all.
Note that it’s possible to buy adapters to fit items that go in small bays
(usually hard drives) into large bays. It is, however, not possible to do the
reverse.