If there is no device /dev/mmcblk0p1 mounted at /home/user/MyDocs this error message is obvious
Just carry on to check the filesystem, because flashing the firmware is not touching the MyDocs partition at all.

There are two scenarios possible:
1) There is no data on the MyDocs partition, then just format this partition, either in Hildon File Manager by long-pressing the device name to get the format option.
2) There could be something valuable data on the partition, then try to repair the file systemReboot the N900 to verify everything is working