Eight drives in RAID 5 all of a sudden won't mount.

Let test the raw disks. The best way to do it is measure the read throughput.

In SoftRAID you can see each disks identifier in the disks tile, like disk3, disk4, etc.

Paste this command into the terminal:

iostat -w 2 disk3 disk4 disk5 disk6

(or whatever your disk numbers are)

then go to SoftRAID and Select all 4 disks and "verify disk".

Each should be doing the maximum reads. This is the fastest theoretical speed your system can do, if you add the 4 numbers together.