Andrei, THANK YOU! Your fix worked. Will this workaround survive future updates to the kernel or udev? I'll add one other observation that could be relevant. Before adding the second disk (when everything was working), the plymouth screen behind the prompt for the encryption password would load very quickly, fading from black to the teal background in 1-2 seconds. Once both disks were added to the array, the plymouth screen took almost 10 seconds to complete this transition. Could that indicate some kind of performance issue related to this? Note that the slow loading behavior persists even after I applied your fix.