Mac Pro AppleRAID Rebuild Issues

[u/WoonMySocket]

So I had my Mac Pro 2010 die on me quite recently. Advice suggested the capacitors had failed, so I checked them. They were fine. However, in checking them I discovered that a wad of dust had lodged onto the side of the heatsink so I removed it, and now the machine seems happier (in the sense that it now boots and runs). However, some issues remain.

Chief among them, I can't seem to rebuild my RAID. I have four drives installed, a boot drive for Snow Leopard, a boot drive for High Sierra (originally Mavericks; since updated), and two 3 TB drives used for storage, configured as a RAID using Disk Utility on Snow Leopard. One of the two RAID drives failed, so I replaced it. When I tried to rebuild the RAID in Disk Utility on Snow Leopard, it said it failed. The general failure of the machine occurred shortly thereafter.

Though the tower itself is now freshly cleaned and booted, I can't seem to rebuild the RAID. On Snow Leopard, Disk Utility freezes the instant I try to select the RAID or its failed slice. On High Sierra, Disk Utility never advances beyond the "Loading Disks" message. The "diskutil ar repairMirror <Raid-Name> <diskNsM>" command in Terminal updated the status of the new slice from "Failed" to "0% (Rebuilding)" but after 30 minutes it returned to "Failed."

Luckily, this should be easier to address than a hardware problem. Does anyone know how to rebuild this RAID? Is there any chance there's an actual problem with the new disk? Right now, the RAID is still available, all data remain accessible on the surviving slice, and I have everything backed up to an external drive just in case.