A590 harddisk distortion

[Amiwest]

distortion could be caused by several things.
RFI -http://en.wikipedia.org/wiki/Radio-frequency_interference
and using Ferrite_bead is used to suppress it. - http://en.wikipedia.org/wiki/Ferrite_bead
but by moving the pwr supply away not helping it is more an indication of a ground loop.
http://en.wikipedia.org/wiki/Ground_loop_%28electricity%29
Plugging computer, hard drive and monitor to the same outlet should cure it if that is what is causing the problem.