Hyper-V virtual disk on external USB hard disk

I was having so much problem with creating a Hyper-V VM on my windows 8 maching, the error I kept getting was

failed to add device 'Synthetic Disk Drive'

This was so irritating, until a colleague told me to turn off my antivirus, and tada! It worked like a charm. He says I can turn it on again after the VM is created and running...