.MOV files not playing after being saved on external hard drive.

I have an old iMac. A 2009 OS X Version 10.9.5. I have so many movies saved in the photo library. I attempted to transfer them to an external hard drive but when I did, they would not play on my new Mac book. “The document could not be opened. the file isn’t compatible with QuickTime Player” is the message I received. How can I get these movies off the old iMac and onto a device where I can access them when I replace my desktop iMac? I tried to open them from the hard drive using VLC and l tried Handbrake to save the files in a different format with no luck.