well, I've upgraded three PSP's already to 3.71 M33 and this is the first time I have encountered this problem. I'm using the "easy install" kits to upgrade the PSP to 3.52 M33, then M33-3 then to 3.71 M33 then M33-2. Earlier, I upgraded this PSP from 3.40 (OE I think.. but I'm sure it was 3.40) to 3.52 M33 to M33-3. Now, when I'm to upgrade it to 3.71 M33, it wont continue and the screen just shows that "this upgrade requires 3.52 M33-3 or higher to continue" and restarts.
I dont know what the problem is, I tried reinstalling 3.52 M33-3 but it still shows the same screen when I upgrade to 3.71 M33...
Maybe there is a conflict in the flash of the old OE & new M33. I would say to downgrade back to 1.50 and then start from there to upgrade back to 3.52 M33 and then 3.52 M33-3, next 3.71 M33 and then lastly 3.71 M33-2. I've never experienced this issue, as I've upgraded from a 1.50. Hope this helps.
Since you're upgrading/downgrading so many psp why not make a Pandora/stick combo with that you can do a straight shot in seconds to 3.71 m33-3 then add 1.50 support (both 1 & 2) i have found that 256MB stick works great and allows for testing of both slim or phat psp