Kobo incredibly unmodificable (is there a ghost inside?)

I´ve got a Kobo Glo for a pair of years, and used it a lot, but it became impossible to modify. Like "freezed" in his actual content, event if it works normally with that content.

It works OK with the books it already has on, but if I delete any file via USB, this files "comes back" as if it had never been deleted, at the next use.

I even tried to delete all the files on the Kobo (even the .kobo system folder), it took 10 mn to delete everything, but, at the next reboot, all the files were here (and also every added file in the meantime were not here)...

I evidently already did a reboot, a reset, a factory settings reset, even tried to actualize the firmware (online and by the "files" way). Without any success...

I´m quite at ease with Windows, but I don´t know anything about Linux (a pity), so I feel a bit abashed.

Did it occur to someone? Do someone have a solution?

Thanks in advance :eek: