So, currently my 1564 is at 2 days and 10 minutes of up time. Seems to me the issue is solved and that VirtualBox was the cause.
I still got "The IO operation at logical block address 6009d0 for Disk 1 was retried." twice in the event viewer in these 2 days. Other than that the event log is clean of errors & warnings. As usual a screenshot is attached.
I'm gonna power her down now to put in the original secondary HDD which had also had a 2.5 day testrun in another pc. Here no errors/warnings were created in event viewer while it was being hammered by VirtualBox pretty much the whole time. (On a windows 7 pro x64 PC) This HDD luckily seems fine.
Now I'll need to hunt down a good VirtualBox replacement!