Dell Precision T7400 (Late 2007) - Manual (Page 255)

Adding and Replacing Parts 255
7 If an empty 3.5-inch drive carrier is installed in the drive bay, slide the
floppy drive into the drive carrier until it clicks into place.
Otherwise:
a Slide the floppy drive into the 3.5-inch drive carrier until it clicks into
place.
b Then, while pressing down the release lever on the side of the drive
carrier, slide the drive carrier into the drive bay until it clicks into
place.
1 floppy drive 2 screws (4)
1
2