OK I finally wen't back to the hotel for lunch and I did what you said:

The files do exist. They are named 20100725-backup-optfs.img (4.19 GB) and 20100725-backup-rootfs.img (257 MB). When I explore the N900 with Windows in USB Mass Storage mode, they are at root (same directory as .documents, .videos, and so on).

The CHKDSK didn't find any error.

As the backup files may be compatible with the Maemo-flasher, perhaps I can restore them by copying them on my laptop and flashing the N900 with them? Too bad I run Seven x64. :/

Maybe it is noteworthy to mention I run an European version of the N900 (Azerty keyboard and so on, I don't know if European N900 are different on the software-side)