I hope this is a typo, since /mnt/usr does not correspond to any actual storage and so would be a new folder created in RAM. All of the unRAID user shares are at /mnt/user
Also looks like corruption in the cache pool
Apr 26 10:37:14 Tower kernel: BTRFS error (device sdb1): bdev /dev/sdb1 errs: wr 0, rd 1, flush 0, corrupt 0, gen 0
Apr 26 10:37:14 Tower kernel: blk_partition_remap: fail for partition 1
Apr 26 10:37:14 Tower kernel: BTRFS error (device sdb1): bdev /dev/sdb1 errs: wr 1, rd 1, flush 0, corrupt 0, gen 0
Apr 26 10:37:14 Tower kernel: blk_partition_remap: fail for partition 1
Apr 26 10:37:14 Tower kernel: BTRFS error (device sdb1): bdev /dev/sdb1 errs: wr 1, rd 2, flush 0, corrupt 0, gen 0
Apr 26 10:37:14 Tower kernel: blk_partition_remap: fail for partition 1
Apr 26 10:37:14 Tower kernel: BTRFS error (device sdb1): bdev /dev/sdb1 errs: wr 2, rd 2, flush 0, corrupt 0, gen 0