I sort of agree with Anonymous Freak, but I can't bring myself to run an old drive rotated by 90 degrees or a modern drive by 180 degrees. There is a little thing in my head saying that it is wrong. The little thing in my head is qualified by comments "about any 1990" or "1985-1990 probably fine". So there aren't any definitive rules or labels for a disk that say "stick me side sideways, if you like".

When the HD manufacturers produce their MTBF tables, what do the tables mean? Have they really tested different mounting positions?

Anonymous Freak: When you manipulate an operating hard disk and experience a torque backlash from it, that is gyroscopic precession.