Parity sync was canceled so can't see the speed issues, also close any other stale browser windows open, and ideally reboot after that, disk2 was formatted so that one will be empty.
If you have 8 disks on this HBA it will bottleneck, about 50MB/s maximum is the expected speed, you can run the diskspeed docker controller test to check bandwidth.
This means the same filesystem was already mounted, cache disk is complaining of a non standard partition layout, could be MBR/GPT corruption, post the output of:
fdisk -l /dev/sdc
Sorry, missed the diags above, you're using Asmedia controllers with SATA port multipliers, these are just 2 port controllers, so bandwhidth is rather limited, see below for a list of recommended controllers:
Looks more like a power/connection problem, but don't have any experience with HP controllers and how reliably they work with Unraid, overheating can also cause issues, those controllers are usually used in servers with very good airflow, make sure there's some direct airflow on/around it.
For some reason that I can't find the 2 new devices weren't added to the pool, it's just 2 devices, hence the capacity, stop array, unassign both new pool members, start array, stop array, re-assign both new members, start array and post new diags.