I read on here a couple of months ago from one of the resident technologoi that the termination supplied by later 50-pin scsi drives is usually sufficient for a IIfx; the black terminator in question dates from a time when termination technology was not built into hard drives. So as I was at the time trying to get a IIfx running, and had no terminator, I gave it a try and I can report that my IIfx definitely works fine without the special unobtanium terminator. Not a single hitch, and running 6.0.8 in "wicked fast" IIfx fashion.
I think the hard drive I put in mine was in the 250-500MB range. If you have two or three old drives lying about, and most of us do, try one and see where you get.
You need good batteries, however, or it will not boot.