Thanks very much for the responses so far all!

@Nix

That's a kernel bug, probably a deadlock. *Definitely* try a newer
kernel, 4.14.3 (the latest) if possible. I bet this is fixed by

I tried updating the kernel, bumped to "4.12.5-041205-generic" first
but still no dice.

@002, @Phil

This feature generally shouldn't be used, because its implementation is
unfinished. Empty BBL's can be removed from every device by giving
"--update=no-bbl" option to mdadm on assemble, but before that you
must manually regenerate content for each block in BBL's and then
manually zero the lists in superblocks.

I am currently running "badblocks -v /dev/sde1 > sde1.badsectors.txt"
and "badblocks -v /dev/sdf1 > sdf1.badsectors.txt" to see what that
comes back with.

How do I "manually regenerate content for each block in BBL's and then
manually zero the lists in superblocks"? This is uncharted (and
slightly scary) territory for me.