i have an external hard drive and all of a sudden when i turn it on, i get a message saying it needs to be formated, format now? and will not let me access it. how do i get around this as it contains very important data and i am very worried i have lost it all!

Try it in another computer. Was it turned off while it wasn't expecting it? Did the computer crash while using it? Be sure to use the "Safely Remove Hardware" function before removing it. That may have been the cause.

I had the same issue with an external USB. Seems the USB chip went south. It was almost new. Try another computer, but if I were you I'd take the drive out of the case and connect it to a regular IDE controller. Be sure to take static precautions and do this very carefully! Additionally, once hooked to an IDE controller you can check the SMART status. (You can use smart tools- free--search on google.)

Additionally, if you still have problems reading your data, you can use a great program called R-Studio. This program is able to very quickly scan unreadable file systems and recover your data. Let me know what happens and if you need help with this.