Comment 2 for bug 873009

I've isolated the fix to 1.99-20ubuntu1 by bisection, using KVM snapshots to restore to a known state before each attempt. 1.99-18ubuntu1 failed in the way described in this bug, but 1.99-20ubuntu1 boots successfully from either disk. (As previously noted, the current version in precise, 1.99-21ubuntu2, also succeeds.)

Although I hadn't been expecting it to fix this particular bug - my intention was to improve 4K sector support - this was almost certainly a consequence of "Support non-512B sectors and agglomerate reads", backported from upstream in 1.99-19. That makes a degree of sense since it's quite possible that the pattern of reads caused by using a degraded RAID array tickled earlier bugs in GRUB's disk cache layer.