Came across a funny problem today, when trying to install an Operating System in an old Core2 Intel motherboard that had embedded Marwell Yukon network cards, and booting it from PXE, then trying to load the image from a WDS server running Microsoft Windows 2012 R2, I was presented with this dreadful error message:

Info: "A required device isn't connected or can't be accessed". As it is typical with Microsoft, the given error offers no significant clue into which direction you should investigate or follow to troubleshoot this issue: you don't know if the device is connected or, if it is connected, if it is accessible or not at all

After much research, I find out that the issue was related to a setting in the WDS server running 2012 R2

Fix: Edit the TFTP properties of the WDS server

To fix this problem, open the properties of the WDS on the 2012 R2 server, then visit the TFTP tab and uncheck the option called "Enable Variable Windows Extension", this will prevent TFTP to negotiate a block size settings with the computers, thus preventing this problem from occurring

References:

Thanks to DXSdata for this great info: http://www.dxsdata.com/2014/07/windows-deployment-services-wds-2012-r2-error-0xc0000001-when-booting-from-image-via-pxe/