PRS-600 Can't put books on the reader

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Hi all!

I bought a PRS-600 last week on Ebay and I am having problems getting it to work.
Everytime I put a new book on the machine, it goes on a loop saying "Loading books" and then the reader restarts, then getting the same message again and this cycle continues until the battery has no power left.

It does this when I try to get books on the machine by using the internal memory and a SD card.

What I've tried so far :

- Putting my PDF and EPUB files directly from the file explorer of my laptop (Dolphin, since I'm a Linux user).
- Using Calibre to export books to the reader produces the same result.
- Using my friend's computer with Windows with the Sony Reader software to no avail.

Could be a memory problem but the notes I'm taking on the reader itself stays in memory so unlikely this is the source of the problem.

Does anybody have any idea what might be the problem?

Sorry for my poor english, not my first language

Thanks in advance.