... but then a black screen with a flashing dash top left. any ideas?
How long do you wait before giving up after you get the flashing cursor?
I get exactly the same thing after I select which kernel to boot from Grub. Like you, the cursor sits in the top left hand corner of a black screen flashing, no disk activity, all looks dead, however... Approximately twenty to thirty seconds later after this period of inactivity the disk cranks up and I get the Mint boot screen with the colour scanned dots before everything bursts into life.
This doesn't bother me too much as I come from an environment where IBM RS6000 computers invariably took about 45 minutes to boot, so I've learned to be patient in this "everything-must-happen-immediately|" world.
If it works, you can install an App called Bootchart which generates a .png graphic chart of which services/processes were running since power on. Useful for identifying what's happening during the seemingly dead period. (No, I haven't got round to fixing my system yet though.) Once installed the charts are kept in /var/log/bootchart.
Hope that helps.