If both Explorer and Opus are doing it, it's probably a 3rd party shell extension that gets loaded into both and is going wrong. (Or something else that would affect both programs, e.g. anti-virus tools.)
You could try disabling shell extensions using ShellExView (fully exit and restart Opus after any changes).
Alternatively, Process Monitor might tell you which files are being accessed when the disk thrashing happens, which might in turn point to a particular piece of software and its extension. (Or the thrashing could be due to low memory and the pagefile being used, which would show up in Task Manager, and could still be the result of a shell extension. There are tools and methods to diagnose that, too.)