Does your eHAU324 still power on?
If it does, I would be inclined to try and extract the firmware from the drive (& the EEPROM just in case).
There is a slight chance that it may have custom firmware. If that is the case, it should be possible to flash it to another iHAS drive which has the same hardware. There is also a chance they might have modified the EEPROM to 'tag' the drive and prevent standard eHAU324 drives from being used.
Also, can you post the full name & firmware version of the drive as reported by CD-DVD Speed, Opti Drive Control or similar?
This may let us identify the drive inside the enclosure. External Lite-On DVD writers often used the same firmware as the internal drive, so there is a good chance CD-DVD Speed will report the name of an internal drive (e.g. iHAS324 B).
The HP drive Matth found (bargain!) should be a rebadged Lite-On iHAS224 B, which can be crossflashed to any Lite-On iHASx24 B firmware.