Please help: Win7 on external FireWire drive results in "No bootable device - insert boot disk..." error

I'm trying to get Win7 working from an external FireWire drive (Macbook pro mid 2009, OSX El Capitan). I've tried the two tutorials below; the installation seems smooth either way and mac recognizes the external hard drive as windows drive after restart (holding option key). But after selecting the windows option, I end up with a "No bootable device - insert boot disk… 1.https://bleeptobleep.blogspot.com/2013/02/mac-install-windows-7-or-8-on-external.html 2.https://blog.macsales.com/40947-tech-tip-how-to-use-boot-camp-on-an-external-drive Would be fantastic if someone has experience with this and could give me a hint! Cheers! Edit: I've now also tried the tutorial here: https://apple.stackexchange.com/questions/225387/no-bootable-device-usb-2-0-macbook-pro-mid-2014/225521# I've adapted the Disk and partition IDs to match my system. Same result, after reboot of the mac the external hard drive is recognized as windows partition, but remains not bootable.