The disk you inserted was not readable by this computer

I had my 1 TB WD hard disk plugged into my laptop and when the laptop hung, I had to force restart the computer. On re-started it showed me this dreaded message-

The disk you inserted was not readable by this computer

with the option of initialising.

Does that mean I’ll lose all my data??

Not necessarily , try repairing on Disk Utility

If that does not work you will need a recovery software 

You can still recover your files, but don’t initialize the hard drive. 

The drive is probably corrupt. You might try a Linux Live CD and boot from that and see if you can access your data. Then there is recovery software Recuva http://www.piriform.com/recuva/download is free and might work. There are also several try and buy programs too.

Joe