Can't SMART two of three drives

I have three HDD connected to a Raspberry Pi running OMV 3.0.76 (Erasmus). On the Storage > S.M.A.R.T. > Settings page I have SMART enabled. On the the Storage > S.M.A.R.T. > Devices page I see my three HDDs. One has its "Monitor" switch thingy showing in green. So that one is enabled. The other two have the toggle in gray and they don't respond to my attempts to change them. The drive that has monitoring enabled, and one of the drives where it is not enabled are the same model drives. And I have checked the SMART data on all three drives before I mounted them on this file server, so I know they have SMART capability.

How can I enable S.M.A.R.T. testing/monitoring on all three drives?

Thanks. All three drives are connected by USB. The one that does have enabled SMART is mounted under /media, the other two are mounted under /src

Thank you. To be clear, it's one disk that does work, and two that don't.
All three are connected to this Raspberry Pi via USB.
The one that does offer SMART is the same model as one of the other two drives that doesn't work.
All three drives so have SMART capability, I've used it previous to mounting them to OMV.
The one that does work is mounted under /media
The other two are mounted under /src (for some reason, that's where they mounted. Go figure.)

So the only thing I can see that distinguishes the one that continues to offer SMART testing, and the other two, which no longer offer SMART testing, is that the first is mounted under /media.

It baffles science.

1. Beats me. Could be.
2. Worth a try. More data = more better ideas.