How to give full access to my other user for all files

Question

When another user logs in to his account, he is unable to access DISKS and other files. He is also admin, but when he tries to open any disk this message shows.

Answer 1

There is this answer...

give specific user permission to write to a folder using +w notation

And this answer...

Change permissions to a specific user in Ubuntu 12.04

...that are very helpful...