Do all rootkit-detection programs operate only on the boot drive of a
given system, or are there some that can fully scan a slaved drive?

And just to be clear, I define a slaved drive as a drive that was the
primary / boot drive from another machine that has been connected as a
secondary or slaved drive to a known/good working system for the purpose
of scanning and analysis.

When it comes to scanning the registry for known signs of malware
presence or exposure, is there any software that can scan the registry
files present on a slaved drive?
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