Recently acquired a Commodore Amiga 500 that included an external A590 hard disk XT. I tried several times to boot the drive without success and although I could hear it spinning up (and other boot-time noises) it never appeared on the Workbench desktop. On the spur of the moment I decided to open the drive up and check connections and review the state of the internals. The XT drive has a old-style head actuator (see in the top right of this picture) with the rotor axle exposed and I happened to twist the axle and I could feel that the actuator was fully extended (the heads were closest to the spindle centre); I had a vague idea that this was wrong. I slowly rotated the axle back to return the heads to track 0. I closed up the case and re-attached to the Amiga, powered up and after several seconds of startup pauses and blinking lights it booted! It has now booted several times without problems.