Permission issues for thumb drive

I have two accounts,1 i am using with gnome and the 2nd with kde.
All my data drives was set to 1st account with gnome.Now i switch to kde with different account.
but now i have flash drive permission issues,mean i cant copy paste to it.i tried to change its permission from that account and as well from root but always giving this error:You have not enough permissions.
I just want to change its group.because i keep that two accounts under one group. But it cant let me do it for flash drive.
Any ideas.
Thanks in advance.

It sounds as if the two accounts are not in the same group; you can check that by finding the group numbers of which each account is a member.