Compaq 247320-003 - Deskpro 4000 - 5166 Model 2500/CDS Reference Manual page 16

The desktop model can house up to four mass storage devices.
Drive Position Configuration
Standard, 3.5-inch 1.44MB diskette drive or LS-120 Drive
Optional PD-CD Drive, CD-ROM drive, diskette drive, tape drive, or hard drive,
(5.25-inch, half height)
Standard 3.5-inch internal hard drive (2.4 GB or 3.2 GB)
Optional PD-CD Drive, CD-ROM drive, diskette drive, tape drive, or hard drive
(5.25-inch, half height)