Just a note to say that after the server disk addition and partition rearranging done yesterday, no more error messages have been logged, nor do any other failures seem to have occurred.
And now there's 146GB of extra space already installed. When I bought the new disks, I found a bargain for four, even though I only needed two. In the next maintenance cycle, I think I'll replace the failing disk with one of the other new ones, leaving me with just one 146GB spare.
It also means that I have two spare 300GB, which I can keep in case one of the disks in the other server fail. Or I can replace its 146GB disks with the 300GBs. This would mean rebuilding the system from scratch, though, as those disks are the RAID pair under the OS. The array of other 300GB disks is used as a giant RAID ZFS store, making the 1.8TB (or maybe 900GB usable) it shares between the database running on that server, and the network mounted storage used by the other servers.