EX4100 array rebuild - drive 1 extremely high usage %?

Hello all, I have a question regarding if I should be worried about a specific drive in my EX4100 or not.

The history is that I have a 4.5 year old EX4100 (24TB / 4x WD60EFRX) that started having performance problems with the drive(s). A full diagnostic showed that drive 1 needed to be replaced, and observation indicated that drive 3 was having major performance issues.

I opted to go with Seagate IronWolf 10TB drives instead of new WD Reds, based mainly on “same day delivery” options, and I’m currently waiting through the final rebuild before expanding the array to 30TiB, but I am concerned with the performance numbers I see.

Replacement of drive 1 and drive 3 went as expected. During replacement of drive 2, though, I saw a notably higher usage% on drive 1 (Seagate) and 4 (WD) than drive 3 (Seagate). Replacing the final drive 4 (now Seagate) is showing 99%-100% usage during the rebuild on drive 1 but only 50% on drives 2 and 3 (also both now Seagate).

Is this to be expected on a RAID rebuild? One drive getting completely slammed? I thought master/slave was a thing of the past, but do not know how the WD SATA adapter works. I’m hoping this is normal for EX4100, but have no problem with RMA-ing drive 1 back to Seagate as “drive ■■■■■; give me another one” if it’s not normal.