Lost data and Pics during transfer.....HELP!

Customer with a Dell XPS 430 model Vista 64 PC is running slow and I went to look at it.She has a 1 TB hard drive that is 85% full. 500 GB of that is Pictures. I got the pc running good enough to bring back to the customer's house while I ordered parts to build a new pc.When at customer's house I backed up all documents, pics, and music to a seagate 1 TB external network hard drive. I told customer not to use the PC while it was backing up. I was told by customer she never touched it since I brought it back to her. After PC parts came in and new windows 7 computer was built I went and picked up the customer's old Dell PC. I then moved all documents, pics, etc. to my PC. When I went to look for data moved to my PC,it was empty. I then looked at customer's Dell Vista PC and data was gone but the hard drive was still 85% full and I still can't find where the data is. It's not on my Windows 7 PC, her Dell Vista PC or the external back up. I don't understand where the data could be if the Dell PC's hard drive is still 85% full but no docs or pics can be found on customer's Dell Vista hard drive. Can someone please explain to me what I can do now to retrieve data.
I also ran a disc defrag and check disc after moving data. If any questions please ask as I am in a bind .
Thanks,
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