This can happen in Windows when you have many "drives" assigned to the same Drive Letter. Windows sux... this is common when you have network drives mounted after you have connected and disconnected your N900.

what to do:
- connect n900 in mass storage
- your MyDocs will not be mounted in windows, dont worry
- go to start, run, and type "diskmgmt.msc"
- find your N900 partition, right click it and choose "Change drive letter"
- select something that is free
- say thank you Carloxic