It is because of the different format now on the hdd. The television formatted the hdd ,while 'registering'
it, with such format that your computer with the format used by 'windows' or 'mac' disk formats cannot read it.
Somebody has said that the format on your disk might be now 'linux format'. If you have got an access to a computer with linux, please try reading the disk there.
You might try as well by using a linux cd, like 'knoppix'. You put the 'knoppix' cd in the optical drive of your computer and boot 'knoppix linux' connect your external hard disk and try if that can read your external hdd as formatted with your television.
I've got a friend with a quite similar tv. I lent to him my hard disk and he recorded something on the disk with the Sony television. I will check on these days with my laptop with debian gnu/linux, if the laptop can now read the file on the hard disk.
