500 gb external hard drive only shows 250 gb

Hello everyone...*sigh*...have a huge problem and would love some advice. I have a Western Digital My Book Essential 500 GB hard drive. My PC runs Windows XP. My problem started when my computer stopped recognizing my External as a Plug and Play Device. When I plugged it into the PC, it tried to install a driver and then would recognize the drive as Initio Default Controller. I downloaded a few small tools and installed and they all recognize the hard drive, but the capacity is shown as 250 GB rather than 500 GB. Can someone help me to solve this? I have some of my most important work on there and I would hate to have to lose 400 GB of valuable stuff. Please ANYONE?

What shows up in Windows Disk Management for the drive? Does it have an assigned drive letter, and is the entire drive shown as the healthy partition?

This provides a good reminder for us all, while hopefully you will get your data out, start backing important data up to something other than just that drive.

I would try some file recovery software before giving up since the files are important. The first that I would try would be Recuva.