Honestly, I'm expecting you to kiss your data on those drives goodbye and take this as a lesson of what not to do with USB. Depending on how bad your corruption is you may suffer anything from a few corrupt files to complete failure of the zpool. Only time will tell.
Well I got
VERY lucky, I woke up this morning to find the scrub had succeeded, and my bkup pool has been restored error-free (no more metadata errors!). (Please note that it appears this is quite rare, so don't bank on it happening to you) :-
[root@freenas] /mnt/bkup# zpool status -v
pool: bkup
state: ONLINE
scrub: none requested
config:
NAME STATE READ WRITE CKSUM
bkup ONLINE 0 0 0
gptid/587327c2-f28c-11e1-a744-441ea13ee656 ONLINE 0 0 0
gptid/5bb7570b-f28c-11e1-a744-441ea13ee656 ONLINE 0 0 0
errors: No known data errors
Also, after doing the following, I fixed the pool in the GUI too :-
-
Detach Volume (in GUI) ---> This caused GUI to hang returned a "pool is busy" error, but the entry disappeared after refresh
-
umount /bkup (in SSH) ---> Somewhere along the way, the drive had been mounted to the root (/bkup instead of /mnt/bkup)
-
zpool export bkup (in SSH) ---> This removes the bkup entry from zpool list
-
Auto-Import (in GUI) ---> This imports the ZFS pool (both USB drives) to the original mount-point and re-adds entry in GUI
Volume Path Used Available Size Status Available actions
hdd1 /mnt/hdd1 1.9 TiB (52%) 1.7 TiB 3.6 TiB HEALTHY
bkup /mnt/bkup 1.2 TiB (67%) 595.0 GiB 1.8 TiB HEALTHY
I think part of the problem was that unplugging the USB drives somehow "pushed" the mount point onto the next available USB device, which just happened to be a partition on the 2GB USB thumb-drive containing FreeNAS. This explains why my second post shows the bkup pool as Healthly with only 823.5 KiB of space, and why it appeared to move to the root directory (or that could have been the "zpool import bkup bkup" command).
So in summary, noobs is right and I totally agree with his rhetoric about using USB drives - I have learned my lesson, and (when finances allow) will move to a more robust backup solution now. But hopefully, my posts will help someone with a similar problem, even when performing a one-time transfer. I have also fixed the GUI by detaching the pool, and auto-importing after the errors had been cleared.
For those thinking of using USB drives permanently with FreeNAS.....don't. :o