it should be a usa update on it then, use that to downgrade.
i think that certain games are pressed before the final specs for that region psp are released so an update may appear that shouldn't really
yes you can use a mac to downgrade a psp, all you do is transfer the files over, no programs are used, easiest way is to use PSPSetX through the eboot loader and then run a 1.5 update
Bookmarks