I thought the A590 used it's own power supply with a round connector; not sure how you could use a A500 supply with it.
What I can tell you is the ROMs on your board are old (Version 6.x), so you are going to have trouble with any scsi drive over 500Mb (iirc 1Gb was the max. limit with 6.6 roms). Your barracuda might simply be too big to work with HDToolbox and the A590. Upgrade to Rev.7 roms.
