3.1.3.2 Drive Letter Protocol Initialization
The server MUST create a volatile Windows registry key where changes will be stored (HKCU\Software\Microsoft\Terminal Server\[session id]\Drive Letter Cache, where [session id] is the decimal RDP session ID). The presence of this key signals to RDP that USB Redirection of Mass Storage devices search a cache of device-to-drive-letter mappings when assigning drive letters to newly redirected devices. The server MUST then send a SADLE_Started message to the client. Upon receiving the SADLE_STARTED message, the RDP client SHOULD reply with a SADLE_SerializedCache message containing the same information as the last received SADLE_SerializedCache message. If the RDP client has no persisted configuration data, then the RDP client SHOULD NOT send a response. The RDP client MUST make no attempt to redirect USB Mass Storage devices prior to the protocol being initialized.