Hi!

Is it possible that inside certain harddisks motion detectors are placed?

Anything is always possible. But I doubt that there is any such thing in a
desktop hard drive. Laptop drives may have such circuitry in place.

Based on some drives that I've worked with over time, I'd say what you're
seeing is likely a result of the spindle motor somehow changing speed
slightly as you move it. The drive's controlling electronics can monitor the
RPM of this motor and may shut the drive down if enough of a change is
detected. The motor may be getting ready to fail or it may have worn
bearings that allow a significant RPM change when you move it.

You can get most desktop drives to reinitialize themselves with a mild tap.
I don't suggest trying that on a drive you depend upon. And while it
generally does no harm to move a running hard drive, you should be careful
not to drop, jar, shake or abuse it while it is running. If any one of those
things were to happen while the drive was running, a head crash or other
drive failure is possible.

William