Update: Initially, I thought this was a hardware issue but a colleague of mine had the same issue on a Dell laptop - I think it's a windows 7 issue and it seems to happen when your laptop comes out of sleep mode. I seem to have resolved the problem by performing the following steps:
- Open Control Panel > Power Options
- Then for your current power plan, click on "Change Plan Settings"
- Click on "change advanced power settings"
- USB Settings > USB Selective Suspend Settings > Set "On Battery" and "Plugged in" to DISABLED.