Moving iMovie (10) libraries to external hard drive
You can just move it, anywhere, then tell iMove to find it again on next launch.
As the library presents itself to the Mac as s single file (even though technically it's a 'special' folder) then dragging to a new location, or Cmd/dragging to move rather than copy, is all that's required.
You can force a new library to be requested if you hold the Opt key immediately after double-clicking to launch. iMovie is capable of switching between multiple libraries this way, if required.

If it doesn't show in that list (which it won't if it's moved) then use Locate… &amp; navigate to it.
It will continue after that to use the new location until told otherwise.
Older versions of iMovie used to keep the original footage separately - so if you see separate folders named iMovie Events, Exports, Original, Projects etc, then move those too. If any of these folders contains a file called UpdatedToiMovie10 then you should be OK without them, but better to check inside iMovie itself before permanently discarding.