On Thu, 7 Mar 2024 14:39:17 -0800
Stewart Andreason [off-list ref] wrote:

$ sudo mdadm --assemble --verbose /dev/md0 /dev/sdc1 /dev/sdd1
mdadm: looking for devices for /dev/md0
mdadm: /dev/sdc1 is identified as a member of /dev/md0, slot 0.
mdadm: /dev/sdd1 is identified as a member of /dev/md0, slot 1.
mdadm: added /dev/sdc1 to /dev/md0 as 0 (possibly out of date)
mdadm: added /dev/sdd1 to /dev/md0 as 1
mdadm: /dev/md0 has been started with 1 drive (out of 2).

Please include "dmesg" output that's printed after running this command.

Where are the Last-updated timestamps? Don't they match?

See the "Event" counters, one drive indeed has less than the other. But that
shouldn't be a problem as you use a bitmap and the outdated parts should
simply resync from the other drive.

As for the actual steps, when you are in this state as in your report, I'd try:

  mdadm --re-add /dev/md0 /dev/sdc1

But to me it is puzzling why it got removed to begin with.

-- 
With respect,
Roman