[Help] BTRFS Error

[Gremlin]

Better late than never I suppose.
What I find is the referenced post is (obviously) old and therefore needs interpretation. Also it seems to assume certain 'defaults' ie md0 and Volume1.
As it happens I found out that

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 lsblk -r
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
sda 8:0 0 5.5T 0 disk 
sda1 8:1 0 285M 0 part 
sda2 8:2 0 7.6G 0 part 
sda3 8:3 0 1.9G 0 part 
sda4 8:4 0 5.4T 0 part 
md0 9:0 0 5.4T 0 raid1 
vg1-lv1 252:1 0 5.4T 0 lvm /Volume2\x0a/var/subvols/8vEbTxkKvwb
sdb 8:16 0 5.5T 0 disk 
sdb1 8:17 0 285M 0 part 
sdb2 8:18 0 7.6G 0 part 
sdb3 8:19 0 1.9G 0 part 
sdb4 8:20 0 5.4T 0 part 
md0 9:0 0 5.4T 0 raid1 
vg1-lv1 252:1 0 5.4T 0 lvm /Volume2\x0a/var/subvols/8vEbTxkKvwb
nbd0 43:0 0 0B 0 disk 
nbd1 43:32 0 0B 0 disk 
nbd2 43:64 0 0B 0 disk 
nbd3 43:96 0 0B 0 disk 
nbd4 43:128 0 0B 0 disk 
nbd5 43:160 0 0B 0 disk 
nbd6 43:192 0 0B 0 disk 
nbd7 43:224 0 0B 0 disk 
sdza 259:0 0 465.8G 0 disk 
sdza1 259:1 0 285M 0 part 
sdza2 259:2 0 7.6G 0 part 
md9 9:9 0 7.6G 0 raid1 /
sdza3 259:3 0 1.9G 0 part 
md8 9:8 0 1.9G 0 raid1 [SWAP]
sdza4 259:4 0 455.9G 0 part 
md1 9:1 0 455.8G 0 raid1 
vg0-lv0 252:0 0 455.8G 0 lvm /home\x0a/Volume1\x0a/var/subvols/8vEbTxkKvwa
sdzb 259:5 0 465.8G 0 disk 
sdzb1 259:6 0 285M 0 part 
sdzb2 259:7 0 7.6G 0 part 
md9 9:9 0 7.6G 0 raid1 /
sdzb3 259:8 0 1.9G 0 part 
md8 9:8 0 1.9G 0 raid1 [SWAP]
sdzb4 259:9 0 455.9G 0 part 
md1 9:1 0 455.8G 0 raid1 
vg0-lv0 252:0 0 455.8G 0 lvm /home\x0a/Volume1\x0a/var/subvols/8vEbTxkKvwa
nbd8 43:256 0 0B 0 disk 
nbd9 43:288 0 0B 0 disk 
nbd10 43:320 0 0B 0 disk 
nbd11 43:352 0 0B 0 disk 
nbd12 43:384 0 0B 0 disk 
nbd13 43:416 0 0B 0 disk 
nbd14 43:448 0 0B 0 disk 
nbd15 43:480 0 0B 0 disk 

provides me with clues concerning the relationship between 'vg1-lg1' and a known volume so that I could put
a "name" to the error messages. (And, even that command in TOS is not "as written in good (linux) book")

I am still unsure (as were others judging by the responses to that post) the relationships between the various /Volume[n] /mnt/md[0] and the exact process to follow, but at least I now have a better start point for experimentation.

And, it still is experimentation. I was able to determine from my 3rd party backup utility that one backup had "ceased to exist" as a viable backup although it was still a file on the drive and being seen by TFM as part of the 2nd tier backup. Likewise that corrupt file was unable to be copied via SMB back to Windows. (Copy failed @ about 50%) The corrupt file has been deleted anyway. I am just hoping that nothing else has fallen over.

Following:
Having determined the Volume (Volume2) I then followed the instructions for BTRFS in the reference post and (eventually) got:

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$:umount /Volume2
umount: /Volume2: target is busy.
{DISABLED REMOTE FOLDER!}
$:umount /Volume2
$:btrfs check /dev/mapper/vg1-lv1
Opening filesystem to check...
ERROR: /dev/mapper/vg1-lv1 is currently mounted, use --force if you really intend to check the filesystem

so, HOW TO UNMOUNT /dev/mapper/..... The instruction I found on-line seem even more convoluted and obscure.