My rough understanding is an SMR drive has a small section that's CMR and where the data goes when you do a write. Then when the drive is idle it moves the data to SMR because SMR writes are slow. If you fill up the small CMR section it starts writing directly to SMR and you see a huge performance loss. Without adaptation for SMR drives a lot of systems recognized this slowdown as a failure and would hault a restore. Even with that corrected for now you are looking at 10x the rebuild time of a CMR drive which increases the odds of another drive failing during the rebuild.
