CD-ROMs mounted internally stick out from the front of the case. The internal mounting depth for 5-1/4 devices is pretty limited, so it's not unusual for the CD drive to stick out from the front of the case. Heat or cable problems can be caused by putting the end of the drive too close to the power supply. Some smaller CD-ROM drives work, notably the Toshiba XM5701B. It may be preferable to mount these drives externally.
Notes on CDROM drive Installation by Willem Schaaij
Some types or brands of CDROM drives are physically somewhat too long. This could lead to a malfunction of the CDROM drive when a hard disk is also placed in the lower part of the Rear Drive Bay (or when a second hard disk is placed in the lower part of it). The power connectors of these drives could then mechanically interfere with each other and physical strain of the connectors, cables, or boards could cause one or both drives to malfunction after some time.
Solution: First, inspect both power connectors and remove any heightenings
on them with a hobby-knife, on the sides where the connectors could hit
each other. Second, place small and thin pieces of cardboard or soft pvc
(1.5 to 2mm) under both ends of the Front Bay, between the bottom of the
case and the Front Bay. The Front Bay will then be lifted somewhat
on the back and will then give more room to both connectors.