Super Duper question?

It could be that your external boot drive is starting to fail, first thing I would do is back up all your data, verify the volume/perms and see if the drive has been corrupted. Returning to your internal boot drive would also give you insight if the external drive was a problem.

If the drives are not the problem then it might be a software problem or OS problem, you may have to re-install your OS.

 

When are you getting a white screen? On boot after the Apple logo and progress bar/spinning gear?

If so, that could relate to either a GPU issue or, more likely, a login item issue. Try booting up into safe mode and see if that helps.

If possible, remove applications from your login items to help narrow down the suspect.

 

Something I'm not clear about.
You wrote above:
[[ I use Super Duper to copy my HDD to an external HD (bootable), and it has set that external HD at my boot drive. ]]

Is there some particular reason WHY you're setting the external to be the boot drive?
Or is SD somehow doing this on its own?
Is there a setting in SD that allows you to choose the external to be the boot drive after a clone is completed?

Do you regularly use the external drive to be your boot drive?
Is this intentional?

I ask because there's nothing inherently wrong with using an "external booter" (I've been doing it for the last two years). The issue is whether you intend this or not.

If you want the internal drive as your boot drive, just re-set it to be so by using the Startup Disk preference pane...