I've got an external HDD "Seagate Backup Plus 1TB". It's partitioned using the GPT scheme and carries two file partitions, an hfs+ OSX partition with OSX operating system installed, and an ext4 partition with Ubuntu on it, as well as a "swap" partition for Ubuntu swap space. It boots fine on a mac.
I've got a more powerful HP Desktop, however, and would like to boot from the there. The desktop is the HP Phoenix HPE h9 Series, bought in June 2012 originally with windows 7, but now has windows 8 pro installed on it.
Below are pictures of everything I think might be useful to whoever might have an answer/suggestion for my problem - which is that the Seagate hard drive is visible in the Boot Options BIOS menu, but whenever I click on it either Windows 8 loads straight away, or I get a "Check cable connection" error, which is in the pictures attached below: