The issues of controller cards, proper cables, terminations, SCSI ID settings and drivers usually mean that we must really need such.
For most we fix the internal drives and use USB connected units.
SCSI documentation is rather plentiful but at first it will be painful to read until you learn the lingo.
My cdrom and cdrw drives stopped working. I bought a new cd drive and tried to replace it and it would not work. Bought a new ribbon and it would not work. I got the idea I could connect an external scsi drive. When it arrived there were no cords. and an attachment with no cords coming from it (I know I sound stupid now) I am assuming this is wireless. Do I connect this to the tower or the unit. If it goes to the tower it does not fit. What am I doing wrong. I think Im over my head.