Inevitably external hard drives will fail. So as an IT professional this is where a provide a reminder: always make sure you have a good onsite and offsite backup plan. That being said when a hard drive is triggering errors on your Windows computer, the predominant course of action is to run the CHKDSK utility.
The CHKDSK utility can be accessed within Windows via Properties, or via Command prompt. Either way if you select the Repair option, it will prompt you to reboot. If you are repairing a System disk, then by all means you should reboot. However for secondary disks, and specially external USB drives, you can dismount them in order not to have to reboot the computer. The dismount option is not available via the hard drive properties, it must be done via the DOS Command prompt. For a complete list of CHKDSK parameters, click here.
For our purposes, the combination of parameters we want to use are:
chkdsk f: /X /r
(obviously replace d: with the drive letter of your choice).
Chkdsk will then force an unmount of your external drive and conduct the repair options while within Windows without the need to reboot. Once finished you do need to remount the drive again.