That's the first thing I thought when I saw he used btrfs. I use btrfs send/receive all the time via SSH and it works great. He could easily have setup SSH serv

That's the first thing I thought when I saw he used btrfs. I use btrfs send/receive all the time via SSH and it works great. He could easily have setup SSH server in GRML live session. There's one caveat though: With btrfs it is not possible to send snapshots recursively, so if he had lots of recursive snapshots (which can happen in Docker/LXD/Incus), it is relatively hard to mirror the same structure in a new disk. I like btrfs, but recursive send/receive is one aspect ZFS is just better.