iMac Boot Camp: No bootable device error when installing Windows 7
You can not use a USB flash drive to install Windows on a 2010 iMac. You need to use a DVD. You can install without a DVD or flash drive, but the procedure is difficult and requires installing third party software.

There are two methods to boot software on your Mac. The older (legacy) method uses the BIOS firmware and the newer method uses the EFI firmware. OS X and many Linux operating systems can be installed to boot using the EFI firmware. Many have attempted to install Windows to boot using the EFI, but doing so in my option is rather pointless. To get all of the device drivers to work properly, you need to install the proper "Boot Camp Support Software". This software assumes you have installed Windows to boot using the BIOS method.

It is my understanding the newer Macs do install Windows to boot using the EFI. All the 2010 models were not designed to do so. Also, the 2010 models were required to install Windows using a DVD. Apple changed the firmware to allow USB Windows installs around 2012, when they stopped putting DVD drives in Macs. To my knowledge no firmware upgrades were ever released to allow USB installs on 2011 or older Macs.

In the case, where you no longer have a working DVD drive, you can still install Windows directly from the ISO file. This requires installing a free copy of VirtualBox. This product can be used to prepare a partition for the installation of Windows. Once Windows is installed, you can remove VirtualBox, but since it requires very little disk space, you are better off just leaving a copy on your Mac.

Instructions to install Windows 7 without DVD (Optical) drive or USB flash drive

The procedure can be found at the link below.

Installing Windows 7 Pro 64 bit without DVD or Flash Drive