Comment 3 for bug 370277

My solution to this same problem:

/dev/sdc or whatever your flash drive is using is owned by user:root group:disk
By default in Ubuntu your user is not part of the group "disk"
As best I can tell, you can't add yourself to the "disk" group with System>Administration>Users and Groups. At least with Ubuntu 9.04 it seems. If anyone knows how, please let me in on it.
Still, what you can do is open Applications>Accessories>Terminal, and type:

sudo adduser s34n disk

! Replace s34n with whatever is your username !

Press Enter and give your password when prompted.

Now you need to log out and then back in again before it takes effect. Or reboot if you'd like.

Perhaps It would make it easier on new users if the error message would recommend adding your user to the "disk" group.