when you mount harddisk as root and not set explicity rights for user ,you won't have the right to acces it.
simplest way is to click on the hd icon on your desktop.
the other way is examine the command 'mount' and and -f options after it.
-f options are : rw,ro,uid,gid,... .
or maybe use 'sudo mount /dev/hdb2 ....'