I am using Ubuntu 12.04 on a laptop computer. I have an external USB disk drive formatted E2fs/E3fs by another linux system (specifically a Linksys NSLU2 running stock Linksys firmware). From the command line as a root user, I have used 'chmod 777' on the mount point to allow me to write around 400Gb of files to this disk. When I move this USB disk drive back to the normal system the new files are visible in a directory listing, but not readable. This is because the file permissions have been screwed up (My background is simple Windows systems, so I know very little about Linux file permissions). The files newly written by the laptop show the owner as 'username'. Older files written by the NSLU2 (the normal intended host for the disk drive) and accessed correctly show the file owner as '501','1000', '2000' etc.
I have tried to use Nautilus to change the file permissions to those expected, but Nautilus will only allow me to select owner names from a pre-defined list, not including the ones that I want. When I try to add a user to the Ubuntu system with the desired user name (to allow Nautilus to offer the correct choice) I get an error message I do not understand - I suspect that Ubuntu does not like these numeric usernames because a bit of research suggests that they are used for special purposes in other Linux systems.
How do I fix the file permissions on this external disk - specifically owner and group names?