jas0nuk posted this interesting information
In 3.80, Sony have heavily "optimized" the fatmsmod.prx which is the memory stick driver, reducing the size by nearly 80kb.
However, whatever they did seems to have had 2 effects
1) Caused crappy read and write speeds for certain memory sticks
2) Totally broken Saved Data and Game menus for certain memory sticks
However, USB hasn't been affected as these drivers haven't changed.
A possible solution is to replace fatmsmod.prx with the decryped one from 3.71 - not easy as the exports will need to be changed to the new random ones in 3.80
Slow but working
Weird occurence: I had someone test a 32MB stick that wasn't working in 3.80 M33-2 after trying all the usual fixes, it instantly worked fine in 3.80 official firmware.
He went back into 3.80 M33-2 and it continued to work.
OK, after collecting this info, looks like formatting it in the PSP under 3.80 usually fixes it. The firmware should automatically format 4GB+ sticks to FAT32, which also seems to solve the problem if done manually in Windows via USB.
Even then, some memory sticks report slowdown with certain ISOs, this can sometimes be fixed by using the M33 driver.
However this is unacceptable, 3.81 should be released with no more stupid POPS bug and a fixed memory stick driver.