The M420 only has 2 external USB ports and is not recommended to run an OS from a USB externally.
The M620 does have an internal USB slot. However, Dell does not support running an OS from a USB key. The only validated and supported method is from the SD card.
Here is a list of validated USB keys from VMWare:
From this list, the Kingston Data Traveler is the only recommended key in our servers.
Regards,