The yet to be released v2.1.0-alpha.8 has a fix which should help with this. Don’t ask when this will be released, it’s been “soon” for months now… Only @mrm knows 
PhotoStructure can now allow drives to go to sleep. It should “just work,” but to set
volumeMetadataTtlMs=0andmountpointsTtlMs=0to force this behavior on platforms that don’t have mountpoint-change-watcher functionality.mountpointsTtlMsdefaults to 0 on docker now, btw.