Thunderbay RAID 5 with 1 failing disk & one out of sync

Hello,

I have a ThunderBay 4 enclosure with a RAID 5 array. Once of the drives is showing an imminent drive failure, and another one us showing "out-of-sync".

SoftRAID XT 5.7.3 (running on a Mac mini with MacOS v10.11.6) has been showing the drive as "rebuilding" for the last 24 hours. But the one drive is still "out-of-sync".

I had tried swapping a new disk in for the one that was failing (ID 0), and the RAID wouldn't rebuild, which is when I discovered that the one drive (ID 1) was out of sync. So I put the original, failing disk back in and the rebuild auto-started.

Then only thing that I can see that shows any activity is the "i/o requests" counting up (slowly!) Is that indicating that the rebuild is ongoing?

What can I do? I'm unable to access the files on the drive... and my cloud backup (BackBlaze) is not up-to-date either.

Thanks!!