My 2012 iMac crashed-actually the switch broke and wouldn't turn on. It was running OS 10.13. I went to a local Apple certified dealer and they backed up the hard drive to a portable back up drive. Now I want to transfer the files to my new iMac 27" running OS 10.15. Migration Assistant doesn't find the files on the WD "My Passport for Mac" drive. I have 50,000 pictures and financial files that I'm afraid of losing but was assured that the back up was good. How do I get them to my new iMac?