what harm can writeback caching have on a degraded disk array?
Solution 1:
From a protection perspective, there is no additional data loss potential from having a writeback cache enabled during a rebuild operation.
Some controllers disable the writeback cache because they don't have enough processor overhead to be able to manage the writeback cache and a rebuild at the same time. Or their firmware is not sophisticated enough to be able to handle both.
There are controllers out there that can do a rebuild while the writeback cache is enabled. You appear to not have one of those.