Hello arnnad and welcome to Seven Forums.
Safely remove the external hard drive using the icon in the system tray. If there are damaged or corrupt system files you could try running a system file checker scan from an elevated command prompt (option two.) If problems are found, run the scan 3 times and reboot the computer after each scan. SFC /SCANNOW Command - System File Checker
If no problems are found you should get a prompt that there were no integrity violations. I would then try to test the external hard drive. If the drive has any important data on it you should save it, just in case. This free utility can scan the external drive for errors. HDDScan