Hi,
The A590 is basically an A2091 inside of an external box IIRC, so you should also check to see if you have the 7.0 version of the ROMs. You most likely have the "Proto" version 4 of the Western digital SCSI controller in there too. That could be upgraded to the faster and less buggy version 8 (or the AMD equivalent) at some point to make your life easier.
You should be able to prep this drive on an A3000 and stick it in there. Make sure you are using a newer version of HDToolBox on either machine and some of these sorts of oddities go away. Adding RAM to either the A2901 or the A590 also helps too.
Good luck!
-G
