Alfa 500mW USB adapters are prone to drop the connection about once a day for no apparent reason. Here is the fix in Windows (less than 30 seconds):
DROPPED CONNECTION REPAIR
Click this icon in your system tray. Click “Support” tab, then “Repair” button.
This is not a permanent fix – it will quickly re-connect the adapter the the network.
A savvy customer told me that the high power of the card itself causes dropped connections. When I find all the technical details that explain how this happens, I will post here.