I want to use an external hard drive (my book for mac) as a backup and as an extra hard drive to store movies and photos and music that I don’t want to place on my desktop imac. Is this possible?
Yes, you shouldn’t have any issues with that procedures.
To store the files, you use copy and paste or drag and drope. And to backup the files, you can use the WD Smartware or the Mac time machine.
I wouldn’t trust any single drive as the only source of important data. You should have copies of the data you wish to move to free up space on your computer backed up on a second drive. The forum is full of people that transfered stuff to a drive as the only copy then had problems with that drive. It doesn’t matter internal or external you still need at least 2 copies to be safe.