Answering my own question, hdparm will make the hard disk spin down, even though it reports an error:
$ sudo hdparm -Y /dev/sdb /dev/sdb: issuing sleep command HDIO_DRIVE_CMD(sleep) failed: Invalid argument -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/466575 Title: secure remove of external USB-HDD produces error To manage notifications about this bug go to: https://bugs.launchpad.net/udisks/+bug/466575/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
