Repartioning External Hard Drive (Mac)

[electric ant]

I've got a 500gb external hard drive which is currently partitioned into 100gb HFS+ for SuperDuper backup and the rest HFS+ for media.

Now I don't really need 100gb for my backup, so I want to reduce this to 80gb.

If I use disk utility to resize it to 80gb, will it affect my data on either partition or is it to ok ?

Thanks