Usb drives are dropped

I can start a backup on my USB drive and it will back up the system but by the morning the backup of every hour will of failed. This started happening after i installed osX 10.11 on my mac mini. When I try to move a file to my external drive I get errors and I cannot move files to that drive. I have tried to unmount the drives by a force unmount and I always get an error message.

I have turned off computer sleep in energy saver preference. This seems to solve the problem.