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How sophisticated are your linux skills?

You could try to check the file system from an external linux PC, be that running installed linux or a Live-CD, IF you have backupmenu installed on the N900. A modern linux has a far advanced fsck application compared to the aging maemo system. This would be my No.1 option.

You could copy all the files from the partition to another linux file system to keep the file permissions, create the file system of the partition fresh and copy the backup to the new file system, but that might also copy the corrupted files.

Well there are some cases also in this forum, where the internal memory simply deteriorates. That would make both possibilities worthless...

For both of these methods you would need to make the partition active