I've been upgrading my 10x 4TB drives to 10x 10TB drives one at a time (raid-z2). The last 4 drives I did two replace drives at a time, while leaving all drives connected (12 drives at once), then removing the old drives. Everyone would be fine and worked very smoothly until the last drives which were getting resilvered and then looking like I had one more drive to replace. I removed the 2 last drives so I had the 10x 10tb new drives, and had a faulted drive, so I restarted and scrubbed...
Upgrading drives now ALL degraded
Upgrading drives now ALL degraded