Please help: Win7 on external FireWire drive results in "No bootable device - insert boot disk..." error
The answer is NO. I you are reading somewhere else that this is possible, then either you are being lied to or the instructions are for a different model Mac.

2009 Macs need to BIOS boot Windows. The Boot Camp Assistant assumes you will be installing Windows by using the optical drive. The 2009 model year Macs can not BIOS boot from an external drive.

You can try to EFI boot Windows 7, but I do not believe you can EFI boot Window 7 from an internal or external drive on a 2009 Mac. 

Apple eliminated the optical drive in Macs starting with the 2012 model year. In order to be able to install a BIOS booting Windows 7, Apple modified the firmware on these models to allow for BIOS booting of Windows from the USB port. This allowed Windows installer flash drives to boot from the USB port. Eventually, this also allowed a BIOS booting Windows to be installed on USB HDD and SSD. (See this question for an example) However, this required installing without the aid of the Boot Camp Assistant.

Apple eliminated the BIOS software from firmware in Macs starting with the 2015 model year. This effectively ended the ability to install and run Windows 7. Current Mac models only UEFI boot Windows and do not even need a flash drive to install Windows. However, you can still use a flash drive if you wish to install without the aid of the Boot Camp Assistant.