Ian, this is probably too late as I only now stumble upon this old thread, but here goes:
I bought a Minolta Multi Pro some time ago from a fellow US photographer and despite it's downfalls as it is indeed an older generation scanner, it does indeed deliver absolutely marvellous quality files from medium format film scans.
Probably the biggest downfall of the scanner is, that in times, it seems a bit picky about a very precise procedure of start up and scanning.
It also seems to be rather picky of connections, software used and the phase of the moon for that matter.
I had at some point a very similar problem, you described - references can be found googling on the net.
It appears, that if the correct startup procedure is not followed or any of the connections is not up to scratch, the scanner will simply not work without giving an easily understandable diagnose of the issue.
I did the following, to get mine running fine on a modern Mac via FW800 connection:
I use the latest Mac OS X and the latest VueScan 64bit software.
I connect an Apple Thunderbolt to FW800 adapter to an external FW800 hard drive, which I use as a passthrough FW800 device, to keep the Firewire connection between scanner and Mac alive even during very long high res scans (this tip - you won't believe it can be found on Ken Rockwell's site).
The scanner is connected to the external FW drive via a standard FW800 cable.
Before switching the scanner on, you have to have this chain established. Don't switch the scanner on before connecting to the Mac !!!
When the scanner is switched on, you have to give it time for it's long startup run - I usually am off to brew a coffee or do other chores, like sorting the film, I am about to scan, preparing a set of neg holders, etc …
Never get impatient and disrupt this scanner start up routing (pulling cables, or worse, starting up the scanning software to early!).
After the scanner has started up and shows the ready LED, I start up VueScan.
This is final check, if the scanner is running and fine, as VueScan will try to find the scanner - if it doesn't and displays the "No Scanner has been found …" message upon startup, I repeat the whole routing.
Only if without interruption this routine has been finished, do I proceed to insert a film holder.
This all sounds very complicated and to be a major hassle, but once this routine sticks, it is send nature and the scanner is fantastic for medium format film.
One tip: for actually scanning 35mm film, I urge you, to try, to get a Minolta 5400 35mm film scanner instead.
It is a lot faster than the Multi Scan Pro and in my experience delivers superior scans in 35mm than the MultiPro (higher effective res, better shadow detail and to my eye a better looking B/W negative scan).
The still reasonable cost, you can get one of these, opposed to the Nikon Coolscan equivalents and the fact, that you can run the Minolta 5400 COMPLETELY automated under Vuescan, makes an additional 5400 a nobrainer (you just have to switch film holders and the whole scanning process is entirely mouse click free - wonderful for bulk scanning 35mm negs).
The 5400 is so good for 35mm, that in fact, I am actually looking for a second unit for a long time now, just in case.
