During last few years, majority of the external HDDs were introduced with USB 3.0 ports which were supposed to make the data transfer faster (& they are until everything is OK). However, these interfaces start wrecking the nerves as the disks become old & a user may come across unexplained errors in the long run. One of them being Windows/OS reporting the drive requires scan and repair to proceed. Most of the times this situation leaves the user panicked & repairing activities may error out without any positive results.
So, next time when you are hit with such an error reported by the OS, give the following a try.
Use a USB 2.0 data cable in the place of USB 3.0 data cable. Check the below images to identify the USB 2,3 portions
Connect your USB 2.0 data cable to the USB 2.0 portion. Check the below image for a USB 3.0 external HDD that is connected using a USB 2.0 cable for data access.
Give it a try, it may get back to your very important data!