Hi All Just got an error message: OS X can't repair the disc

Hi All

Just got an error message:

OS X can't repair the disc 'Samsung'.You can still open or copy files on the disc,
but you can't save changes to files on the disc.
Bring up the disc & format it as soon as you can.

(The Samsung is the external HD I am using for backup).

So I went to Disc Utility to try & fix things (I've had to do this twice already in the past year or so).

Halfway through the process I got another error message:

Restore Failure.Recovery partition restores can only be done on GPT partition maps.

So my Samsung external HD is now wiped & OS X won't let me restore from Macintosh HD.

Can anyone advise me on how to proceed please.

Many thanks.