Copying recordings off and back on after reformat will appear to work and all Standard Definition recordings will play. But High Definition recordings remain encrypted when copying off onto USB, and I believe a reformat of the HDR Fox T2 hard disc creates a new encryption key which will mean HiDef recordings copied back on won't play.
This problem can be avoided, but discussion of decrypting HiDef recordings is not permitted on this forum.
I suggest you go down the custom firmware route to fix the *Deleting problem.