The jumper (XT/SCSI) is actually only for the activity LED :-D
About using large(r) drives in the A590 it depends (just like with the A2091) on the EPROM version (7.0 recommended) and the version of the WD SCSI chip.
Sometimes drives appear to work, but as soon as you partition the upper part of the disk it will overwrite previous partitions!! So always do a full format on all your partitions, reboot and check if they are still there!
