Dell OptiPlex E1 - Manual (Page 59)

When you replace the 3.5-inch diskette drive, be sure that the 2 hooks on the right side of the bracket engage the mounting holes in the side of the drive.
5.25-Inch Drive Assembly Removal
To remove a 5.25-inch drive assembly from the middle or lower drive bay, follow these steps:
1. Disconnect the DC power cable and the interface cable from the back of the drive.
2. Press in on the 2 retaining tabs (1 on each side of the drive), and slide the drive assembly forward to remove it.
3. Remove the 4 screws attaching the 5.25-inch drive to the mounting plate, and lift the drive out of the mounting plate.
When you replace the 5.25-inch drive, align the front of the drive flush with the tab at the front of the mounting plate. Insert the 4 mounting screws, and tighten them in
the order stamped on the bottom of the mounting plate.