Did you connect the N900 to the PC while switched on in Maemo system and did you export the device in 'mass storage mode'?
The problem is the setup of Maemo itself, because it does not care for file system corruption and will always just create these folders in MyDocs, even if the partition is not available. This causes testdisk to produce this error, because of the exact same names of the folders.

Please install 'BackupMenu', it is capable to export the device and all of the other partitions to your PC without Maemo running, and thus not interacting with the export in any way.