Migrating library from Mac to Linux

Searched the forums, but didn't find a working solution, so I thought I'd ask this here...

I'm moving my Calibre library from my old Intel based Mac to an Ubuntu box. I exported the library data from the Mac and copied all the files it to the Ubuntu box, but when I try to import it, I get this message:

The folder /home/calibre/IMPORTME is not valid: The first part of this exported data set is missing

Traceback (most recent call last):
File "site-packages/calibre/gui2/dialogs/exim.py", line 257, in select_import_folder
File "site-packages/calibre/utils/exim.py", line 262, in __init__ ValueError: The first part of this exported data set is missing

I copied the 100GB of files over without a hitch, so, what am I missing? Are the library exports not compatible across Mac / Linux?

Thanks for ANY help you can give me on this!