[23:17][minijack:~]$ sudo btrfs check --readonly /dev/sdb1
Opening filesystem to check...
warning, device 2 is missing
Checking filesystem on /dev/sdb1
UUID: 08e51c76-0068-45ba-bac8-9c1f57363ec6
[1/7] checking root items
[2/7] checking extents
[3/7] checking free space cache
failed to load free space cache for block group 9658368917504
failed to load free space cache for block group 9660516401152
failed to load free space cache for block group 9661590142976
[4/7] checking fs roots
[5/7] checking only csums items (without verifying data)
[6/7] checking root refs
[7/7] checking quota groups skipped (not enabled on this FS)
found 2982890913792 bytes used, no error found
total csum bytes: 2908627664
total tree bytes: 3749707776
total fs tree bytes: 430489600
total extent tree bytes: 115179520
btree space waste bytes: 399181399
file data blocks allocated: 2990515580928
 referenced 2987045777408