BootCamp doesn't work (no bootable device found)

Hey guys,

I use a 2011 MacMini with a 250 GB Samsung 830 SSD.

I'm trying to install Windows via BootCamp and at first it seems to work. It allows me to partition the SSD, create a bootable pen drive with an ISO (I've tried with three different ones), but after reboot it says 'No Bootable Device found'.

How do I fix this? I tried three different ISO-files: one original Windows 7 64 BIT, one original Windows 8 and one pre-activated pirated copy of Windows 7 (that's the one that worked on my MBA, by the way).

What can I do to get around this?