zzz reports `zzz: another instance of zzz is running` with mounted external hard drive because `/sys/power` cannot be locked

[MIvanchev]

I have a setup with a 2TB Intenso HDD connected externally and when the disk is mounted, zzz will report

sudo zzz
zzz: another instance of zzz is running

Unmounting the disk through a /etc/zzz.d/suspend script has no effect because zzz never gets there, it fails to get the lock on /sys/power. Unmounting the disk prior to executing zzz allows the lock to be acquired.

Changing the script to

echo "yawn."
) 9>/var/lock/zzz.lock

always works and there are no issues. I also do not understand the very specific choice of /sys/power as a lock target.