Adding files to a backup drive

I’ve acquired a WD MyPassport 1 TB external drive (a USB plug-in which appears as SYSTEM RESERVED G: and H:). I have a Windows-7 64 bit machine which is also equipped with a 1 TB WD internal drive (C:). Using Acronis True Image, WD version, I was able to clone the W-7 64 internal drive (C:) to the MyPassport USB drive. Question: Neither drive was even close to being full, there’s still more than ½ TB not in use. Can I copy additional files from another machine onto the external USB drive without messing anything up should it ever become necessary to restore the internal drive on the W-7 64 bit machine? Does anybody know? Thanks.