Just thought I would note an issue with the latest changes to usbd in the sdk. One of the more recent changes prevents the usb_mass driver from compiling now.
build-imports.c:75: warning: data definition has no type or storage class
A temporary fix is to use the older usbd.h in ps2sdk/iop/usb/usbd/include. Just place it in the ps2sdk/iop/include directory then usb_mass will compile successfully. I know the changes are relatively new but I figured I'd give someone the heads up.