okay, i'll clear some stuff up.

X-flash is alot better, because it is bug-free, and it verifies the flash write, so if anything goes wrong, it will restore it.
PSPSet is buggy, and freezes quite often during flash writing, resulting in a brick.

To test that a gameboot wont brick your PSP, you can replace the gameboot file in a firmware dump on DEVHOOK, and then emulate that firmware. Then, try launching a UMD. If it freezes, and you get a black screen, you know that the gameboot is a bricker. If not, and it displays correctly, then passes on to the game, you can be 99% sure that it is safe.

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