Why does my computer want me to format my external hard drive when it worked fine for the last few years?
Most likely the Master Boot Record of the drive has a problem.
If you originally setup the drive with a CD furnished with the drive, it may have maintenance options that include restoring the MBR.
If not, Windows may help you.
Right-click "My Computer"
Click "Disk Management"
Expand to full screen
Look for that drive in the right-pane of the windows.
If it exist, right-click in the area showing that drive.
Click "make drive active" if this option exist.
If this does not work, get data recovery software that can read through a bad partition and copy everything out you want to keep,then format the drive, copy everything back.