If a project has MIDI Sync Output to ON saved in system settings, then you won't be able to load it with success. Check this. If it's ON, turn it to OFF, write and load the project again.

I have one USB drive which also refuses to load the Mega Pack, however, trying to set the Sync Output to either ON or OFF doesn't help. I am inclined to think this might be a RAM issue, not USB... but have no spare DIMM to check this.

Just bought a 4 GB drive from Apacer and it works fine with the Mega Pack, loads it in under 10 minutes.

Got a second Kingston drive with 4 GB space, and it doesn't work with the Mega Pack just as my previous Kingston drive with 1 GB.