I'm really sorry for this, because I've seen quite a few different posts about this, but none of them seem to solve my issue.
I have a relatively old Sony laptop, running Ubuntu 13.04, with the "Mobile Intel® GM45 Express Chipset x86/MMX/SSE2" driver.
When I plug in my external monitor (Samsung SyncMaster T240) via HDMI, the laptop freezes entirely. I can't move the mouse and keyboard is unresponsive. The only thing I can do is a hard shut down.
Any help appreciated.