My 640g drive is only showing 292g of available space

WD External Drives WD Portable Drives
1

Where has all the rest gone and how can I get it back?

2

Since there’s been no replies, maybe you should try contacting WD’s Technical Support about this. You can do so either by phone or email.

To Contact WD for Technical Support
http://support.wdc.com/contact/index.asp?lang=en