I've made a bit of a balls up...
When attempting to create a bootable Ubuntu usb drive I accidentally ran dd on my main system drive :(
sudo dd if=/path/to/image.img of=/dev/rdisk1 bs=1m
I realised what I'd done pretty quickly but everything seemed fine so I re-ran the command on the correct drive /dev/disk2 and thought "phew not sure how I got away with that"... Then things started going wrong... I could continue using the applications I had open but opening anything new / accessing the hdd caused that thing to lock.
Sadly I haven't been using time machine, lesson learnt!
I spoke to AppleCare who said I need to reformat the machine but I'm really hoping there's some way to restore whatever 1m I destroyed (I'm assuming something to do with the partition information). Wishful thinking?
Also, AppleCare told me to restart, which means I can only user repair mode utils.