SSL 2 is USB bus powered and will turn off when the computer is off or the USB is disconnected.
Some computers have an always-on 'charging' USB port, and if this port is used the SSL will stay on regardless of computer power. This type of port may be denoted with an electrical symbol.
It is ok to use the charging port and leave the SSL 2 on, but if you want your SSL to turn off for electric or environmental reasons, you will need to not use a charging port or simply disconnect the USB.
Some always-on ports can be disabled in BIOS.