I have Windows 7. My Freegate external hard drive runs via a Belkin 4-way USB hub shared with my Epson printer/copier. When printing, I am continually asked to format the drive. It contains 30 days worth of music and many other files, I do not wish to lose these.
If your external hard drive is USB-powered, the issue is probably that your external harddrive is short off power when you are printing. This is because USB ports can only deliver a maximum amount of power (which can vary depending on which USB port you are using). From practical experience, I've noticed that Windows suggests to format a hard drive when it doesn't get enough power.
(Check the following superuser question for determining the maximum power per port on your computer: How do you check how much power a USB port can deliver?)
If my above assumption is correct, you can solve your issue by making sure your hard drive gets enough power all the time by:
If the answers provided by others do not help fix the issue, then the answer is simple: make a scan disk, leave it to scan and fix issues. Most probably there are some errors, corrupted files that need fixing. The warnings should stop. If you do not know how to make such a scan, check this tutorial for guidance:
If the warnings continue even after you do this, most probably you have some real hardware issues with the drive. In that case, you should copy all your data to another drive and take it to warranty for repairs.