so I found an instruction on support-en.wd.com
Upgrading Drives on 4 Bay My Cloud (RAID 5) - which is what I have 4 2TB Drives in RAID 5.
Went to the MyCloud Dashboard, clicked Storage, RAID, Change RAID Mode, OK on Warning Box, Clicked on Expand Capacity, Next, asked do I want to replac eyour existing drive with a new dirve YES , Next, then Finish after a final Expand Capacity screen.
At this point it said to Replace the Hard Drive with the Red LED. It showed RED led on drive one, I removed, it installed my new 4TB CMR drive, and it started “expanding the volume” it said.
I left for a few hours, come back and it was ready for the next drive, Red LED on drive #2. So I installed 2nd 4TB drive. It continued to expand the drive, I went to bed.
The next morning I come to check on it, and was surprised to find that the Red LED was lit up on drive 4, skipping drive 3. (showed 70% complete) So I pulled drive 4, installed 4TB drive in slot 4. It continued and finished expanding as it said. At completion It says the RAID is healthy , drives all healthy, with 3 4TB dries, and 1 2TB drive in slot 3.
So what do I do now? The PR4100 is working normally but with no realized increase in capacity. It never asked to change drive #3. ??