the disk you inserted was not readable by this computer

[ukp1959]

I can't get my 1 TB Seagate Goflex external HDD to work.
I have had this drive for 10 days , and it has worked fine until this 22nd dec 2011.
On 23rd I copied so many video and stills to that drive. while trying to see files ,the hard
whenever i plug it into my computer I get a " the disk you inserted was not readable by this computer" error message. it then gives me the options to''initilize "ignore" or "cancel". it shows up in disk utilities as "3.9 GB SEAGATE USB 2.0 CABLE Media" but unlike my other external HDD, (VIDEO-3), the devices name "Media" does not appear underneath the technical name
external HDD IS POWERED BY USB 2.0 .

" the disk you inserted was not readable by this computer" message.
THIS DISK WAS FORMATTED IN MAC
I AM USING IMAC WITH FOLLOWING CONFIGURATION:
Model Name: iMac
Model Identifier: iMac8,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.66 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 6 MB
Memory: 2 GB
Bus Speed: 1.07 GHz
Boot ROM Version: IM81.00C1.B00
SMC Version (system): 1.29f1


All advise is a great help!

 

[iMacC2D]

Removed.

 

[robgendreau]

Have you opened Disk Utility and run disk repair on it? If not, do so.