NSA325 v2 Degraded - How to Fix?

Hi,

I have an NSA325 v2 with 2x WD Red 4TB NAS HDDs in RAID 1. Recently, the NSA started making a loud whirring sound while running, and sometimes would not start up or shut down on the first attempt (I'd need to hold the power button multiple times).

I've had a look at the Web UI and it reports that the volume is "degraded". Also, only one of the disks was appearing as part of the volume. I've removed both disks in turn and run an extended SMART test on them in Ubuntu, and both disks passed with no errors. I've also tried running the NSA with only one of the disks inserted, and it seems to work fine with either disk although it reports the volume as "degraded".

Looking at the disks in the Web UI, the S.M.A.R.T. package shows they are both healthy.
I'm looking for ideas on what to do next to try and fix this? The NSA's "repair" function is available, but I'm not sure what it does. Should I back up my data before trying it? Are there any other tests I can try on the HDDs to make sure they are OK?

Since noticing the weird whirring sound, it now seems to have stopped and no longer happens with both drives inserted.

Thanks :)