Didn't you say had hardly any data on this HD? Dayam.
I meant to clarify that as "hardly any data worth keeping" - i.e. personal documents, it still feels the need to scan my 80GB collection of legitimately acquired MP3s, the MSDN content installations and of course, the tens of gigabytes that Microsoft Windows likes to amass in the WinSxS directory.
It's now 2011-11-06 00:40 and it's just hit 70%, so far it's found 4 threats, I note that for as long as I've been watching it it's been scanning my Firefox cache - so if the malware installer found its way through there it makes sense.
I'll post a follow-up when I check back in the morning.