[TrueLoonix]

You dont need a stick with MagicGate,
just one you trust

The Custom firmware installer will check the update files for Integrity (an MD5 check i think) so that should not be the problem.

I dont know thou if the finished fused Cusom update is checked again for coruption while writing on the MS so... use one you trust ^^

edit: Maybe its a good idea to test the MS by copying a large .rar archive to it.
rar archives are checked for integrity and Winrar will give you an error message if any data inside the .rar file ot corrupted.
its just an idea, i have not testet this test ^^