Installing windows 7 on Imac I have a mid-2010 27" iMac with macOS 10.12.5 and an external DVD - USB SuperDrive (the internal DVD drive is broken).  

I can't install Windows 7 from DVD with Bootcamp. After downloading support software to USB flash drive via BootCamp Assistant, I get an error on restart:


  No bootable device -- Insert boot disk and press any key.


What can I do?
You need to convert it to an ISO. 
Apple's official guide