Ah yes, all part of Sony's annoying limitations for the Vita memory cards. It's not enough that they cost so much for what they are (£15 will buy you a 64GB SanDisk microSD, whereas you'd need to spend about quadruple that for the same size Vita proprietary one), they're also locked to a specific PSN account and firmware version. Unless you have two Vitas on the same firmware and same PSN account you basically can't reuse memory cards. It's a dumb, dumb move. I've read stuff that talks about removing the id.dat file but I wouldn't want to try it myself.