I'm assuming your laptop is Windows?
I guess I'm the only Windows guy here. lol

In general, dirty is a term used for NTFS file system when it has a dirty bit set due to a non-clean dismount of the volume. So if your MyDocs partition which is FAT32 on the N900 is said to be dirty, it can be cleared by running "chkdsk /f" on that drive from the command prompt. (or right click properties on that drive from Windows Explorer and go to tools - error checking - check now and make sure to check the box for auto fix)