I have heard that it is generally considered unsafe to power off a mounted hard disk; the consequences could be a lost or unresponsive drive. However, powering off a drive after unmounting should be no problem.
I am using hdparm to spin down my disk automatically after two minutes of inactivity. The drive spins down but is still mounted - and accessible, too. If, for instance, I issue the command
ls /media/external/drive/ the shell hangs for a few seconds until the disk is up and running again.
My question is: Is it safe or unsafe to power off a drive in a "mounted" AND "spun-down" state?