You can also get one of those SCSI2SD adapters. I have one in my A3000 and it works like a charm. It's faster than the HD, and it's silent, since it doesn't spin.
If you scsi controller has the external port on it, like Mike mentioned, there is even a small, 3D printed case you can put the SCSI2SD card into, and, an adapter you can buy to make it external (a 50 pin internal SCSI to 25 pin external adapter). It doesn't require power other than what comes across the SCSI cable, even though it has a MOLEX.
