I have one catalog but I have it backed up on a portable drive. The primary catalog is on an external drive at home. When out of the house can I make changes and modifications to the catalog on my portable drive then sync up those changes to the primary external drive later?
Yes, you can make changes to the catalog on your portable drive while travelling and later on merge that catalog with the main primary catalog on your external drive.
Chetna has you pointed in the right direction for working with a master and "travel" catalogs.
Alternatively, you could put your images and catalog on the external drive and just move the drive from computer to computer as needed.