@matigo sdb failed utterly. Dead. Kaput. Replaced with new drive. Now, it turns out, that sda has errors. SMART reported nothing. Never once got a degraded raid event until sdb failed. I should have gotten one the moment sda started developing errors. Unless, of course, it picked JUST NOW to develop them.
I'm forcing a sync as best I can onto the new drive, booting from it and replacing sda too.
I may wind up having to put a bare drive in, cpio the filesystem over, go through the gyrations of making the new drive bootable and a raid device and going from there.