@s.mazzucato. Oh..
1. Try another forum for your question. You weren't necessarily 'wrong,' to put it here but it might be very worthwhile to copy & paste it and put it in the iMac section and in a Time Machine section, as well.
You're in the OS Lion 10.7 section, which is fair enough but current 'going crazy' with the latest update issues and your problem might be better to be in the section specific to your type of computer, not the OS --- since you're problem has been from Day One, as you mention, 'all previous versions of Lion,' -- from the day you first tried the external drive.
This problem is not unusual. My external drive has both USB and Firewire and where and how you plug these things in, and to what type of machine or hubs they are going through, can cause problems and less than perfect performance.
Less than perfect performance sometimes happen and sometimes you have to plug and unplug and try this and try that. Lots of times the issue is not the OS, or not even your computer but just something unique to how the user has set it all up or certain incampatibilities between different brands of equipment (or apps).
I think you may find a solution to your issue. It is certainly an unnecessarily annoyance and it would be nice -- and we all learn something -- when we try to iron out such difficulties, bit or small.
Good luck.