u might be able to delete everything manually with vitashell. the app folder is likely the largest one since that contains vita games. u want to keep pspemu though. I personally erased my memory cards, then sold them since they're so expensive. personally, I'd try removing the sd2vita cart, then seeing if you can format the card. I know that this is possible, because I had my sd2vita 1.0 cart not making a secure connection once, which forced a format of the memory card. I can't recall if it was being read as uma0 though as I never checked.