user # access?

Running Ubuntu 16.04 updated. I have a few disks from my older linux
comps, even way back to Mandriva :-), and I'd like to check for some
info on those disks. When I try to access them, I can't. Says owner is
user# 501, etc and I don't have any permissions. Tried installing
Nautilus-admin but that doesn't work either. How can I access my old
disks via a GUI so I can easily see all and copy some if necessary?
These will not be used to boot, so I don't mind altering those disks
if necessary. All have different users/passwords/OS if that makes a

Tried a few I knew were from Mandriva - looks like they all use 500+.
Not good - I used MD for quite a while and have several
disks. :-(( Liked it a lot at the time, though.