Comment 1 for bug 121900

This is likely to be a permission problem. Many filesystems commonly used on flash drives (like vfat) do not have support for file permission/ownership themselves. On modern distributions like Ubuntu, fake permissions are put on top of these filesystems in such a way that normal users have read and write permissions.

Ext3 on the other hand does have full support for unix style file permissions (usb drive or not), and these will be respected by the operating system. As a result you may well have insufficient rights by default to write to the filesystem as a normal user after formatting. Please mount the usb drive and then change the permissions and/or ownership in order to make the drive writable (using sudo when needed).