他のことをやるときは、いったん切断していた

$ sudo mount /dev/sdb2 /mnt/sdb2

$ sudo mount /dev/sda2 /mnt/sda2

$ sudo btrfs sub list /
ID 4457 gen 312984 top level 5 path @home
ID 4458 gen 312984 top level 5 path @
ID 4475 gen 312975 top level 5 path timeshift-btrfs/snapshots/2021-12-03_00-00-03/@
ID 4476 gen 312977 top level 5 path timeshift-btrfs/snapshots/2021-12-03_00-00-03/@home

$ sudo btrfs sub list /mnt/sdb2
ID 278 gen 50 top level 5 path @
ID 279 gen 52 top level 5 path @home

$