I have owned several MDD beginning shortly after the first model was released...
Firewire ports and Target mode were always flaky.
Wow, never heard that one, let's clarify, FW Disk mode on an MDD 2003 FW400 Single or Dual will work 100% of the time, I literally used to do that to prep dozens of machines back in the day. Now as for FW800 MDD, always loaded OSX via optical, so maybe an issue ?
BUT: Target mode will work ONLY with hard drives connected to the IDE ports.
Yes... kinda makes sense, you are turning the entire mac into an eternal FW have drive, so connecting other items to it, like other FW drives will not work in FW target mode; as far as internal SCSI drives, they use a card with it's own BIOS, so again will be invisible; items need to be connected to the logic board when turning you mac into an external Hard drive via FW target. As a tech, this feature has "saved the day" too many times to count.
Seems like Metropahge may have alluded to this problem with the FW800 MDD back in 2016:
http://macos9lives.com/smforum/index.php/topic,3242.msg20625.html#msg20625Lastly, since nanopico said:
Target disk mode doesn't rely on the OS, it's entirely in firmware.
The way to fix this issues on an MDD 800 may be to burn the FW400 firmware on a FW800 MDD, this is what many did BEFORE we made our famous boot disk for the FW800. Somebody should PM a member with a FW800 unit running FW400 firmware and see if target mode works. Also, if resetting the PRAM of a FW800 MDD that has not been modified and trying Target mode does NOT work, then that would make no sense since our modified ROM does not alter firmware at all, so a PRAM reset should put the unit back to "normal"