RAID's are generally rebuilt online. Which means that the normal read and write activity is still occurring. Rewriting a block of data in the middle of a SMR region requires reading the entire SMR region, updating the changed data and then writing it back. So, normally a raid controller will just stop updating a strip (frequently much much smaller than a SMR region) and perform whatever forground activity is needed. The smarter ones rebuild the the strip of the new data along the way. This is going to really mess up rebuilding on SMR because suddenly what you assume is probably a linear operation isn't from the perspective of the drive.