Have a PR2100 with 2 x 8TB. One drive threw a bad drive warning light. B&H had a good price on a pair of 12TB Red drives, so I decided to upgrade. The new drives arrived Wednesday afternoon. I pulled the bad drive and inserted the new one. Upon restart, the rebuild started.
It is now Friday morning. Rebuild is at 39% and has been since sometime yesterday. Also, the power button LED is red. The web UI shows a “failed firmware upgrade” as recently as this morning. Apparently, I had automatic updates on. I’ve turned that off.