gsmartcontrol. SATA samsung disk. It is an external disk that I can connect with USB and eSATA, and the error also appeared with eSATA. Now, the the disk does not react when connecting with USB: perhaps the error lies in the circuits in the case, I will try later to connect it with eSATA.
camcontrol standbyfor the disk. This stops (i.e. spin-down) the disk in a controlled way, which is better than forcibly removing power by unplugging it. See the camcontrol(8) manual page for details.