Solution: No Bootable Device Bootcamp error on an iMac using USB
A short video explaining the reason behind the "No Bootable Device" error message on a iMac (Pre late 2012).
If your iMac has a built in optical drive (all iMac's pre Late 2012 will have this), Apple have disabled the ability of booting from a USB. Instead you will need to burn your Windows 7/Windows 8 to a DVD-R or DVD-RW.