I know this is an old post and thread, but I came across it due to a similar issue in which my old PC finally went.
The drive was fine but the almost 10 year old motherboard decided it was done. None-the-less, I was looking
for an easy way to get the Windows 7 drive booted in Fusion on my intel based MacBook Pro.
After trying a few things, like the suggestions here. I restarted Fusion (v. 4.1.3) and realized I could use the Boot Camp VM.
It was smooth sailing after that. It boots the USB drive, installs the VM tools and drivers needed.
Don't know if you could go back the other way, but in this case, I don't think I will be doing that.
Hope this helps someone down the road. The how to for Boot Camp is in the help screen for Fusion.