The A590 had both a SCSI and an XT header for internal drives (on the right side):
http://amiga.resource.cx/photos/photos/a590-5.jpg
I only ever saw them with SCSI drives in person, but they also sold XT units. Maybe you saw the latter?
http://amiga.resource.cx/photos/photos/a590-5.jpg
I only ever saw them with SCSI drives in person, but they also sold XT units. Maybe you saw the latter?