I already installed Windows 10, using the Boot Camp Assistant, on my MBPr (late 2013) without any problem. However, it is a "new" model, everything went fine.
But, now I am trying on an iMac (late 2009) and the process is a little bit harder.
The Optical Drive was replaced by a SSD, so I needed to change the Boot Camp Assistant to accept ISO images]1. Now, the Boot Camp Assistant creates the Windows USB flash drive installer.
But, on boot, the flash drive is not visible. If I just wait, the message "no bootable device" appears, so I guess it went looking for the Optical Drive – so I restart one more time, pressing ALT so I can select the flash drive and... It is not there! (Just some "EFI something")
If I insert the flash drive on my MBPr, it can find it and Boot as it should be, so the 'problem' is just on iMac.
Ps: I am using a Windows 10 ISO, that theoretically has no drivers for this iMac – but I think that this is not the answer for the flash drive do not appear.