Dell OptiPlex Gxi - Manual (Page 109)

When you replace the 3.5-inch diskette drive, be sure the two hooks on the right side of the bracket engage the mounting holes in the side of the 3.5-inch diskette
drive.
5.25-Inch Drive Assembly Removal
1. Disconnect the DC power cable and the interface cable from the back of the 3.5-inch diskette drive.
2. Press in the retaining tab release button and pull the drive assembly forward to remove it.
3. Remove the four screws attaching the 5.25-inch drive to the mounting plate, and lift the drive out of the mounting plate.
5.25-Inch Drive Removal