creating an imagef an existing drive on a wdPassport without wiping out the eisting files on the Passport

I used an earlier free version of TrueImage-2013 and was able to create an image of a lap top drive. I was also able to add other data files and backups from a second laptop onto the same WD drive. The first laptop harddrive failed a month later. ||They installed a new harddrive and the image was restored successfully. I updated the data files and now wan to create a new image. I was so impressed with the free version I decided to buy the latest 2015 version to use on all three machines |I own. When I went to create the new image on the WD drive I could only Clone the drive and if I went ahead and did that then all the existing files on the WD would be wiped out. What am I missing here?