Use my external drive for backup & xxxx?
 

SuperDuper erases my drive before the full backup. That's a shame because it has 340GB of space remaining afterwards. If it put anything there, it'll be gone next time. Or am I missing something, is there a way to use the external drive for backups and whatever else...

Yes. Partition the drive into a backup volume and another that you can store data on. There's a whole chapter about this in the User's Guide...

partition
 

Thanks, will do.

Absolutely true. A file doesn't really exist unless it exists in two places. :)