[zamfi]

Huh, which process was doing this grinding on your MacBook?

[cguess]

In my experience this is usually Spotlight indexing files/checking previous indexes etc. If you have any backup software running that will do it too.

[vbezhenar]

Generally it's mds_stores, but sometimes there are other ones which I don't remember atm. But mds_stores is the biggest offender.