PR4100 upgraded 4x2TB to 4x4TB, skipped drive 3

so I found an instruction on

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. ??

Because you have a PR4100 you may want to see the dedicated OS5 subforum for the My Cloud Pro Series where users more familiar with the PR4100 may be able to assist, and where you can use the site search feature (magnifying glass icon top right) to search for past discussions that are similar to your issue.

OS5 My Cloud Pro Series

Some relevant WD Knowledge Base Articles if you haven’t seen them already.

How to Upgrade the Drives on a My Cloud Device (RAID Expansion)

My Cloud RAID Online User Guide and Solutions

Perhaps, after backing up any user data if any, try running through the expand capacity option again pulling out each of the drives when indicated and reinserting them. This time do it when you have enough time to watch, or keep an eye on, the entire process (i.e. not when your going to bed). And replace the 2TB drive with the 4TB drive if and when indicated.