On 2018-10-06 07:23 PM, evan d wrote:
> I have two hard drives that were never partitioned, but set up as two
> independent BRTFS filesystems.  Both drives were used in the same
> machine running Arch Linux and the drives contain(ed) largely static
> data.
> 
> I decommissioned the machine they were originally used in and on
> installing in a newer Arch build found that BRTFS reported no
> filesystem on either of the drives.
> 
> uname -a:
> Linux z87i-pro 4.18.9-arch1-1-ARCH #1 SMP PREEMPT Wed Sep 19 21:19:17
> UTC 2018 x86_64 GNU/Linux
> 
> btrfs --version: btrfs-progs v4.17.1
> btrfs fi show: returns no data

Did you try a btrfs device scan  ?

(Normally, that would be done on boot, but depending on how your arch
was configured, or if the devices are available early enough in the boot
process....)