Unmounting /usr from an external drive [closed]

Question

I mounted /usr from the nvidia TX1 dev board to an external SSD connected to the board.

I am wondering how I can restore it original state without re-flashing? If I power down,and disconnect the SSD, and start, there will be no /usr directory.

I was thinking of making a copy to /root/usr and updating fstab to point to that instead of the external SSD, but there has to be a better option, I just cant think of it at the moment.

If it were a regular x86 I'd just boot to a live CD and fix it, but this is an SoC with Arm, so it's not quite that easy

Answer 1

Use a bind mount of / to make the original /usr (which should probably be empty there if /usr was mounted over it before /usr was ever populated) available and copy the mounted /usr over it.

# mkdir /root/underlyingroot
# mount --bind --make-private / /root/underlyingroot
# cp -ax /usr /root/underlyingroot
# umount /root/underlyingroot

--make-private is to cancel the case where / is mounted with the shared option, which it is when running systemd. Else anything mounted (eg: automount of an inserted device etc.) between the mount and umount above will be reflected inside /root/underlyingroot and prevent the simple umount /root/underlyingroot working after.

Now that the copy is done at the final place you can edit /etc/fstab and remove the /usr mountpoint.

If nothing at all running is using /usr you might be able to also umount immediately /usr and be done. But nowadays it's hard to have things running not using /usr at all if not in single user or rescue mode and today not even even always (eg newer CentOS), so a reboot is probably needed anyway. You can also consider umount --lazy /usr which would allow to immediately get rid of /usr and have any new updates to /usr done on the internal storage instead of external, but external would still be required until next reboot.