Hi there, have answer from Hasselblad
"To the nature of your query: Firewire ports need a constant 10W of power generating into the digital back in
order for it to function correctly.If you do not have this you can run into intermittent errors such as the one
that sounds like your experiencing from lack of constant power.
In fact, many computers supply less tan the 10W required as specified in the manuals and other documentation. This can be effectively helped by adding a mains powered FireWire hub between your computer and your camera system. I believe these run about $100 - $125 and the one we have had the most success with has been the one from Granite Digital. I am afraid we have had no luck at all using the one from Gefen, so I'd stay away from that one for this purpose anyway.
You can find the hubs at most pro dealers and at:
http://www.granitedigital.com/index.asp?PageAction=VIEWPROD&ProdID=146"
hope this can help