You definitely don't need to rebuild your sets from scratch.
Change the name of your new external drive to match the name of your old external drive. Then, all the paths should end up the same. This is probably the easiest and safest option, and may also solve problems with other softwares' projects.
Make a "symbolic link" from /Volumes/olddrivename to /Volumes/newdrivename
If for some reason changing the drive name is problematic, e.g. will break a bunch of newly saved projects, this allows you to make an alternate name so that both the old and new names work. However if you ever upgrade your drive again, you will have to remember to recreate the link so that both names work.
I seem to recall that M8 will search for source files relative to the project file. So if you have a disk structure like:
it will find your clips.... even if the clip was originally /Volumes/bigdrive/random/path/clipa.mov
But, definitely test this with one file before you reorganize your whole hard drive!!!
Edit the .mp8 project files and update the paths. This might make sense if you are switching to some new organization system which is better than what you used to use. The project files are just XML, so you don't have to be a super hacker. But, it is still easy to mess up your project files forever, so only do this if you have a good reason, are confident, and have a backup of all your project files.
I think I have used all these methods at some point. Good luck!