Well, if you swapped two hard disks in the controller and you got the same behaviour, maybe the fault is in the Mtec controller.
I had a similar problem with my A500+ and an Oktagon 508 SCSI controller: after some months of unusing, I starup the system again, and I got a bad red screen, reporting serious problems.
Without controller everything was fine; disconnecting the hard disk gave me same results.
But by pushing the controller/moving it a bit, the behaviour changed (always no boot, but no red screen).
I investigated and I discovered that the problem was related to some oxidation on pins of some ICs inside the controller (that are socketed); all of this despite I kept my Amiga and controller in a good environment (no moisture, strong humidity, dust, etc).
Hope you can solve!
Ciao,
Luca "OgniX" \8^)