My Passport shows space being used but no files

I have a My Passport for Mac (1T).
I used it to back up my old MacBook Pro back in 2016.
I got a new MacBook Pro and when I tried to connect My Passport to it it shows there is about 350GB of storage being used but I cannot see any of the files on it.
I have some important files and documents on it that I want to retrieve but do not know how.
Could it be that the OS on my new macbook is newer so it cannot read the old backup? Or what could be causing this? I got a message after running Disk Utility’s First Aid saying “Problems were found with the partition map which might prevent booting.” but no other message prompting me to repair it. I urgently need help guys. Thanks in advance (:

You should check after repair or reformatting of partition map in this concern. But recommends to take a backup of important files before going to format as it’s a data destructive process which can’t be undone.
Or you may contact data recovery if data is not accessible.