That shouldn't have anything to do with it locking up in my opinion. The G Manual plainly states that you can choose to have the Fantom G boot up in either Live, Studio, or Single Mode and Roland doesn't mention any problems associated with the G booting up in some other Mode besides the factory default Live Mode that I'm aware of. I haven't read the Manual cover to cover yet so I may be speaking prematurely regarding this though.
But you should be able to boot up the G in any of the three modes if you wanted to and still be able to load samples or whatever on boot up in any of those modes and be okay I would assume. I wonder if anybody else has experienced this lock up on boot up in Studio Mode with their Fantom G?? If so, then it would be more of a confirmation that there is a problem and that people need to load samples (or whatever) in only Live Mode on boot up possibly. But I suspect the problem is not whether the Fantom G is booting up into Studio Mode, but rather, how the Fantom G is setup in the System Menu regarding USB storage settings on boot up, etc., or possibly the thumb drive itself may be the cause.