Actually, you're in luck. When my external HDD died and NCSX wouldn't support the warranty on it, I opened it up myself. It's an internal IDE HDD unit without the rails. It's plugged into a circuit board that goes to the wires for the PCMCIA card connection and the power source. There's also a jumper cap with wires coming out of it that goes to the switch on the bottom of the external case, to switch between Master and Slave settings, although Slave is never used with the PS2.
When I looked at the expense of replacing the HDD unit, after determining that the hard drive stil functioned but a circuit inside the casing that the drive attached to had fried, I instead sold my model 10000 and replaced it with a model 30000 and a Network Adapter, and it has been functioning to this day. Therefore, the external casing is just an adapter to hook an IDE hard drive up to a PCMCIA interface.
Unfortunately, I no longer have access to he casing and circuitry of the external unit. I mailed it off to someone in Australia a while ago since they thought they could fix it, and I didn't know about the custom Sony HDD firmware at the time.
My PlayStation2 HDD guide.
NTSC J SCPH 30000 with HDD and PSBBN 0.32
NTSC U/C SCPH 30001R Astral Blue LE PS2
Final Fantasy XI status:
Server - Ragnarok, Race - Mithra, Jobs - BLM 72 WHM 51 SCH 39 SMN 25, Nationality - Windurst, Name - Bungiefan