There is no way that Windows 7 can show files that Windows 8 can not. It is simply not possible.
==> That doesn't mean you have a certain problem that needs to be solved. So I will try helping you ofcourse
To find a sollution to your problem I have some additional questions.
* How do you copy the files to the external HDD?
* What is the extension of the files you copy to the external HDD?
* Are the files DRM protected?
* How is the external HDD formatted? E.a. NTFS/ FAT32/ etc.
The way, in my honest opinion, you should move the files from pc A to pc B is this:
* Open Windows Explorer and browse to the location on pc A where the files are stored.
* Copy the directory of the files and browse in Windows Explorer to you external HDD.
* Paste the files onto your external HDD and disconnect the HDD.
* Connect the HDD to pc B.
* On pc B open Windows Explorer (in DESKTOP MODUS!!!) ==> you should see the files!
This MUST work. So, don't copy through third party software like Media Player, or iTunes. Just use Windows Explorer.
Please let me know the answers to my questions and try above work-description. I hope this will help you solve your problem.