Epson 3865X Plus - Manual (Page 226)

2. There are two holes on each side of the disk drive. When
you insert the drive, you align the appropriate round holes
in the drive with the oval holes on the side of the drive bay,
as shown below.
3. Insert the drive into the lower drive bay and slide it toward
the back of the subassembly. Watch the oval holes on the
side of the drive bay so you can see when the holes on the
drive are positioned in the middle of the oval holes on the
subassembly.
If you are installing a diskette drive, adjust its position so
that the front of the drive lines up with the drive in the
upper bay. (A hard disk drive fits all the way into the drive
bay.)
Installing and Removing Disk Drives B-35