Yeah, that happens to me often. I've the adaptor Photofast with 2 4gb microSD, and sometimes it get on read only.
There's 2 options here.

1- Copy all you can, and format (still, sometime soon, it will do that again)
B- Turn the USB on, copy/delete/move/edit something, then get read-only. Turn USB off, Turn USB on again, copy/delete....etc etc. (this is so crappy, but works)
[ ] - Also, if you have a card reader, you can take the card off, put it in you pc, do what ever you want, put it back in the PSP, and then profit.