You RPI will always boot from SD card. But there is a way to use only the first stage of SD card and then load the root filesystem from Hdd. In this way your RPI will work as PC with the Opsys installed in HDD. There are more that one way to do this. I think this is the easiest way: You have to write the image to both your sd card and your HDD. Then remove the first partition on the HDD (the fat32 one). Change the /boot/cmdline.txt to point it to /dev/sda1 (on the sd card - fat32 partition). Expand the ext4 partition on the HDD to your preferred size and you should be good to go. You can either remove the ext partition from the SD card or leave it there as a backup. That's all.