I've fixed that link so you can d/l the RAM TEST disk
here. Thanks for reporting it.
Yesterday I typed up some useful info about pinouts, dip switches and jumper settings for the a590 found
here.
This also contains info regarding your SCSI question and configuring the RAM.
Correct me if I am wrong but the only limitations for SCSI HD size is that of the Amiga, info about that can be found
here.
Found the a590 manual on my website too,
here. [.zip] I have the [.lha] if need. [2.5mb]
Have fun with your a590!