Win7 keeps telling me to format a working external HD

I have an external USB HD (WD Elements) that I've used for back up for over a year. I moved last week and after setting up my system Windows 7 occasionally makes the ejecting sound and tells me I need to format the drive before using it. That said, Windows sees the drive and I have no problem accessing my files.

I did format the drive two days ago and backed up to the drive again in hopes this would solve my issue. It did not. I'm still prompted to format multiple times a day even though I'm able to see and access the drive.

What the heck is going on?

Craig