I needed to mount it as a --bare FS, ascertain what its ID was with lsblk, then create a directory to mount in (sudo mkdir /mnt/test), then mount it there with sudo mount -t btrfs /dev/sde4 /mnt/test.

Hello, I got this strange behavior while filling my new RAID 6 from a backup (3.5" hard drive in USB 3 dock): Versions and topology $ command rsync --version rsync version 3.4.1 protocol version 32 ...