Hi there and welcome

What this generally means is that some of the data on the disk has become corrupted. Windows sees this and knows that the data is not usable (for it) and asks to format it so that it can be used again.

The fact that Windows cannot read it though does not mean the data is not recoverable

What you need is a recovery program that can recover "RAW" files and convert this back to usable files (if possible)

Then best tool for this is this one here

http://tinyurl.com/86arywa

The tool is not free (none of them are) but you can use the trial to see what can be recovered.

The website for the tool is here

http://www.z-a-recovery.com/download.htm

You do stand a good chance to recovery your files though, but this tool is really the last resort. If it can not recover everything, than you will need to consider the remaining files lost.

How did this happen?

Most times this occurs when you do not properly eject the disk from the computer. It is very important to do this, as you will avoid this situation.

The instructions for in the future on how to eject the disk properly from windows can be found here

http://tinyurl.com/4vsbrw

It is important you do this every time before unplugging it from the computer. If you shut down your computer while it is still connected than you don't have to do this.

If you require any additional information at all, please do not hesitate to let me know.

In advance, Thanks for giving a positive rating! Ratings are how I receive credit for helping you today.

If you need further assistance for this problem before or after rating feel free to ask a follow up and I'll be
glad to help.

Thanks,
Richard