I just had the experience of upgrading a 3850 switch stack. This type of switch has a USB interface which allows for easy(read: faster!) file copying.
As usual when upgrading switches, you can do it in two ways; file copy or archive. However, with a switch stack it was slightly different. We need to make sure that each switch has the same SW version. And apparently is doesn’t work when the image is copied directly to the flash. This must be done directly from the USB.
Using archive requires the *.tar file!