I have not found it to be an ownership problem as on my Elyssa system the locked file from the DVD will still list my userid as the owner of the file (i.e. NOT root).
My experience has been that the reason for the lock is that the file copied from the DVD has retained the read-only permissions of the DVD. Makes sense, a file on a typical DVD-R should be read-only, not read-write.
On the file in question, right click on it and in the menu that opens select 'Properties". Click on the Permissions tab and at the top you should see the owner listed (your userid) but the access will be Read-only. Click on the Access combobox and change the Access to Read and Write. The lock will disappear.
This seems to be an annoyance in Gnome burning software (at least on LM Elyssa). Both Brasero and Gnomebaker share this...neither retains the original file permissions when burning a data disk. K3B has a (somewhat buried) setting in the Burn dialog box to keep file permissions (backup). Thus I use K3B on Gnome!
All progress requires change, but not all change is progress.
--Henry Steele Commager
...there are some...who use change to promote their careers. And then there are those...who use their careers to promote change.