More information about my problem:
I installed the nightly build from 2006/11/22. Could someone @ rockbox team try it on an iaudio x5, repeat the few steps described in my first post and see if they have the same problem?
I tried to format the drive yesterday evening, with bad blocks verification enabled. It was not finished this morning but it showed a _lot_ of bad blocks. It's very strange, how can a drive fail like that? the bad blocks start ~ at block 30400 and every 128 block fails.
The regularity of these bad blocks seems very strange, i think it could be a problem with USB, perhaps the cable or the way rockbox handles USB (but i remember reading somewhere that it was handled by hardware?).
If i had money to waste (~€370), i'd buy an adapter, open the x5 and try the disk on a real PC, but i don't want to loose my warranty.
Now, I just want to remove the bootloader and restore original firmware so I can return the unit to the shop.
Any help would be very much appreciated.
Thanks