After the helpful email from both mdadm and smartmond (enable in /etc/default/smartmontool). I replaced the drive, per the fine mdadam manual.
All went well, until for fun, I decided to check if the server would boot with only half of /dev/md0 (mounted on / ) available. The server didn't boot.
After some head scratching, I realized I hadn't installed grub in the MBR of both discs. The RAID array (mirror) is defined in terms of volumes , or partitions, not discs. To install grub on both MBRs I had to:
Though it would have worked as well to:
sudo dpkg-reconfigure grub-pc
Once Grub was on both MBR, all was good.
sudo grub-setup /dev/sda sudo grub-setup /dev/sdb