An update, I got a regular end terminator, so now my SCSI chain looks like this:

Mac <-> DB-25 to CN50 cable <-> External HD <-> CN-50 terminator

However, now when I try turning the Mac on, it gives the chimes of death. When I used the pass-through terminator it would at least start up, although the drive was not recognized. I tried different combinations of terminator placement (top or bottom CN-50 connector on the back of the external HD) with no luck.

I noticed there's a number ticker on the back of the external HD, presumably the SCSI ID. It is set to "6" currently. Does that make any difference in terms of the computer recognizing the device?