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
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: CMD(sleep) failed: Invalid argument
issuing sleep command
HDIO_DRIVE_