Exile360 -- Thanks for the prompt and informative response. If I understand you correctly:
>>...if you are using a custom scan then Malwarebytes should be scanning where you tell it to.<< [Emphasis mine]
There is no practical way to verify that this is actually happening; and
>>Malwarebytes targets active threats via an advanced heuristics detection engine rather than using traditional AV style signatures/database... ...Malwarebytes will be very unlikely to detect any inactive/dormant files that might be on your secondary drives.<<
the frequent updates taken by Malwarebytes are not signature files, as I had supposed, but something else; and "dormant" files (which I interpret to mean potential malware waiting to be executed rather than malware that is actually running) will not be recognized by Malwarebytes until/unless they are executed. Unfortunately, this last conclusion (if correct) seems to imply that they will never be recognized by the free version, which does not have on-access scanning. (At least I seem to be using the correct approach with the free version of combining it with another, signature-based, on-access scanner.)
Am I basically correct, or is my ignorance still blinding me? -- JCW2