write caching is a feature that is enabled by default in windows .
This feature helps improve disk performance. External hard drives when idle, spin down and when windows tries to get access, temporarily will not be able to and it throws a fit, giving you all those warning about delayed write failed.
To solve this, you can go to the device manager , under "Disk Drives" go to the external drive and double click it.
Then go to the Policies tab and select, "Optimize for quick removal".
That will disable write caching and all the problems related to it.