I've got an old Western Digital ext hd that uses this power supply (http://www.amazon.com/Adapter-Western-Digital-Wdbaau0010hbk-01-Supply/dp/B00FPLZFP4).
I was planning on powering RPi 2 just from the USB port on my Samsung LCD TV.
Will there be any issues (eg backfeed) if I also mount the powered ext hd?
There should not be any problems. The RPi 2, like the B+, improved the power delivery to USB ports. Also, if your hard drive is powered from the mains it is going to draw even less power from the USB port. As long as the drive is plugged into the USB port and running before you switch on the Pi there will not be any problems. So in theory, the USB port on the TV would supply adequate power to the Pi.
However, my recommendation would be to invest in a proper Raspberry Pi power supply which are specifically designed for powering the Pi. This is probably going to be a better option than powering the Pi from a USB port, which should be avoided.
Also, this page may be useful to you: Powered hubs, backfeeding and safety
