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#11
Hard Disk Sentinel
I also have not gotten into Storage Spaces but looking at the images I see 2 x WDC/Western Digital 1TB hard drives and 1 Seagate 1.5TB drive for a total of 3.5TB. The 2 WDC drives are the same model but have different Serial Numbers.
You could of course remove the disks and test them individually if you have appropriate hardware.
Another approach could be to use Safe Mode and test them individually from there, or even by using a live boot disk.