Please provides some details about the vSphere version you are using, the AutoDeploy mode, the server vendor/model and the target (e.g. RAID controller) you are using. Depending on the RAID controller model/driver it might be required to add some additional options to "first-disk".
Did you have any success with this? I am having a similar issue with a Cisco UCS B200 M3. I am using Autodeploy with Stateless Caching. The target device is a mirror of the the two UCS SD cards. They are presented to the host as a usb device, so far I have tried esx, local, usb, and usb-storage without success.