27" iMac - No bootable device - Standard solutions tried, no success

Details: 27" inch imac, 3.4GHz, i7, 16GB RAM, 1TB HDD, late 2012, running OSX 10.9

What we're trying to do: Partition the drive (200GB) for a bootcamp to install Windows 8.1, 64bit, using Casper Imaging

How far we have gotten: Using Bootcamp to create the partition via USB flash drive, we remove the drive after the partition is complete. The Mac reboots and we hold "N" to get the iMac to boot to our LAN, so we can install our version of windows. However, the Mac leaves a white screen for close for 45 - 60 seconds, and then we receive the error message "No bootable device - Insert boot disk and press any key".

We have checked the support forums and these are the things we have tried:

Holding the "alt" during boot - no change.

Tried "Command Option + R" (Recovery Boot)

Reset the PRAM - no change.

Someone suggested holding down Command + Option and pressing "R,T,Y,U,I,O" respectively (Never heard of that one before)

Because our windows installers are on the network, we normally have no need for installing from the local drive. We have tried to boot to "local boot drive" on the USB installation media, but it does not respond.

Currently I have 3 (three) identical iMac's, all with the identical problem.

Thanks in advance for your help.