The disk you inserted was not readable by this computer.

I have a Seagate Freeagent Goflex Desk External Drive 3TB and it was working fine a week ago, but when I go to plug it into my Macbook Pro 15in mid-2010 model I get the following message:

"The disk you inserted was not readable by this computer."

with the options to "initialize", "ignore", or "eject".

I searched the forums & saw a few people had some answers, so I tried using disk utility & terminal to restore the GPT headers, but I got this message in terminal when I tried that.

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sudo gpt recover /dev/disk3s3

gpt recover: /dev/disk3s3: no primary or secondary GPT headers, can't recover
DNA-Imagery-MBP:~ DNA$
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Does anyone else have any suggestions on what I can do or what the next step will be? I have important files I need to access & I can't have my external harddrive down like this. Any help would be much appreciated.