Just another slightly related experience. When switching back from OS2008 to 7 with N800 you can try to flash also older bootloader when having trouble with MMC, not just flash os2007 kernel/initfs/rootfs over newer full flash. For me it made difference for external slot. When going back to OS2007 my external mmc slot did not work until I flashed bootloader. It did work with OS2008 kernel but not with OS2007 one until I did that. The internal one worked always, though.