Before I purchased my new Mac I backed up all my files onto a external hard drive. When I copy some of these files back to the new Mac I can't open them? I double click and zip-nada. I tried opening them on my old mac - I get this error message? I also cannot open them directly from the external hard drive.
Help? I'm using ID version 15.1.2
And is it the same problem if you do a File > Open and browse to the .indd file? How was your external hard drive formatted, btw?
Did you run First Aid on your hard drive to help rule out incorrect permissions on files, including files and/or subfolders in your new Mac's InDesign application install?
You'd have been far better off using Dropbox and/or Onedrive for this purpose. You could have signed in on the new computer and the files would have synced automatically. Do you still have the old machine? It's not too late.
I don't trust external drive and I certainly would never attempt to work from one.