how do I narrow down the scope to just C: or D:? The disks are single volume, no raid array etc..

If C: and D: are two different disks (as you used plural "the disks"), you just need type this command below to see which one is OK and which one is Pred Fail:

wmic diskdrive

The output would be something like below:

Availability  BytesPerSector  Capabilities    Caption                  Description  DeviceID            Size           Status     StatusInfo ....
              512             {3, 4, 10}      Hitachi HDS721010XXXXXX  Disk drive   \\.\PHYSICALDRIVE2  1000202273280  Pred Fail             .... 
              512             {3, 4, 10}      ST3500XXXXX              Disk drive   \\.\PHYSICALDRIVE1  500105249280   OK                    ....