Error reading NetCDF files from external hardrive, but not from Mac.

I am getting an error when I attempt to read netCDF files from my external hard drive (My Passport) from my iMac:

However, when I move the files to my iMac I can read them no problem.

I would like to keep my large data files on an external hard drive so they don't use up space on my iMac.

Can anyone explain what the problem is and how to solve it?

Thank you!