On 21/05/2020 13:33, David T-G wrote:

% Can't remember the syntax, but there's a re-add option. If it can find
% and replay a log of failed updates, it will bring the drive straight
% back in. Otherwise it will rebuild from scratch.
%
% That's probably the safest way - let mdadm choose the best option.

OK; yay.  I'm still confused, though, between "add" and "readd".  I'll
take any pointers to docs I can get.

Add just adds the drive back and rebuilds it.

Readd will play a journal if it can. If it can't, it will fall back and 
do an add.

So *you* should choose re-add. Let mdadm choose add if it can't do a re-add.

Cheers,
Wol