"Flasing the eMMC resets the MyDocs folder contents to factory settings. N900 users don't need to reflash the eMMC of their device. If you are reading this it's probably because you got a pre-production device e.g. in the Maemo Summit. If you have a sales unit and you have problems with your eMMC you should contact Nokia Care.

Also note that any backups created in your MyDocs area will need to be copied off the device to be safe across an eMMC flash as the MyDocs area and other partitions on /dev/mmcblk0 will be erased.

Note that the eMMC images available do not contain pre-loaded maps. If you reflash your eMMC you will lose them (the Maps application will work as usual but you will need to download the maps needed). " - Firmware update wiki