i think this is possible. swithc off the phone, and then plug it into the laptop. that is what i do sometimes and both the drives in phone (mmc and hdd) gets mounted automatically (i am running F12)

The problem is that to unmount the eMMC you need to unmount swap, /home and /home/user/MyDocs which might be tricky. I think while the N900 is running, it simply will not let you unmount /home. Also even if you could, you would still then need somewhere to backup to and as the eMMC is as big as the largest microSD card, you would be talking a samba/NFS share or something which is going to be pretty slow unless you do it over USB networking, which is overly complicated.

Basically, I was thinking more about backing up in the same way you flash, without the OS running at all. It must be possible as I find it hard to believe you can flash but not read back the contents, its just the flasher command probably does not support it so we would need someone to write a backup application. I'm quite surprised someone has not done so already as being able to make a full backup of the N900 would be insanely useful.

That only mounts /home/user/MyDocs from the eMMC, I am talking about backing up a full image of the eMMC like the official Nokia recovery image, which contains 3 partitions, swap, /home and /home/user/MyDocs. Likewise if you could backup root flash too you could make an exact image of your N900 when configured exactly how you want, very useful for playing around with extras-devel. It would allow you to recover your N900 very quickly to the same state it was when you did the backup, no waiting for all your applications to download again.

I ran into an error much like this recently (Pro Tip: don't put keychain in .bashrc or .bash_profile like the desktop install guides say; you will receive much pain and suffering), and was able to fix it using the MeeGo rescue initrd tools: http://wiki.meego.com/ARM/N900/Using_Rescue_Initrd

The "recovery terminal" mentioned there is largely valueless (the keymap's incorrect, so you can't type anything that requires the blue arrow shift key, which includes such vital characters as / and -), but that setup + USB Storage toggle + a Rescue Is Possible bootdisk I keep with my laptop allowed me to get to the broken .bashrc that was keeping me from booting.

(There's newer versions of that initrd image and kernel floating around in those repositories, but I haven't tried them to see if the keymap is fixed in those.)

Alas, I only discovered this after I'd tried reflashing the rootfs, so I still have some recovery to do. But at least I didn't have to wipe my eMMC...