I was running Bitdefender Anti-Virus free and found that on boot (Windows 10) it my external USB drive would show as usual but show an I/O error if I tried to open or create files.
If I then unplugged the USB cable and plugged it back in, it was absolutely fine.
I worked out it was Bitdefender, and by fully uninstalling it the external drive works perfectly as soon as the PC boots now.
Before I uninstalled it I tried disabling the autoscan functions on flash drives etc.. but it still had the issue. Has anyone else had a similar problem or know the setting to use to allow the external drive to work on boot up?