What file system is the new disk formatted to?
If it's NTFS (which is more than likely, if it's a modern external USB drive), ubuntu will only mount the disk as read-only because it doesn't yet fully support writing to NTFS out of the box.
It is possible to try out the latest NTFS-3G release. Here's a tutorial for ubuntu edgy eft:
http://howtoforge.org/ubuntu_edgy_eft_ntfs_ntfs_3g
In my experience it's mostly stable, but v. slow.
Peter.