I am using a 2011 Mac mini as a backup server for all the data I store on iCloud, and for the first few days while I was setting up the Mac, I noticed the 4 TB and 2 TB external USB drives I had plugged in would spin down after a few minutes, and I would have blissful silence as long as there wasn't an active operation on that Mac (which should be fairly rare; just hourly Time Machine backups and periodic SSD activity since the iCloud libraries are all on SSD).
However, after a few weeks, I noticed that at least one of the two hard drives runs continuously, 24x7. Something on the Mac mini must keep hitting the drive and preventing it from spinning down.
To see what was happening, I used sudo fs_usage | grep VOLUME (in my case, VOLUME is 4\ TB\ Utility) to monitor what processes were accessing the drive, and what files they were accessing. After a few minutes watching (and doing nothing else on the computer, to make sure I wasn't causing any extra filesystem seeks), there were a couple regular culprits:
10:52:13 getattrlist /Volumes/4 TB Utility 0.000005 lsd
10:52:13 getattrlist /Volumes/4 TB Utility 0.000004 lsd
10:52:37 getattrlist /Volumes/4 TB Utility 0.000006 lsd
10:55:32 stat64 es/4 TB Utility/.Spotlight-V100/Store-V2/069E7AE9-4CA8-4342-AF0A-EE472A279FC4 0.000007 mds_stores
10:55:33 stat64 es/4 TB Utility/.Spotlight-V100/Store-V2/069E7AE9-4CA8-4342-AF0A-EE472A279FC4 0.000019 mds_stores
10:56:21 statfs64 /Volumes/4 TB Utility 0.000009 Spotlight
10:56:21 getattrlist /Volumes/4 TB Utility 0.000009 Spotlight
10:56:22 fsgetpath /Volumes/4 TB Utility 0.000017 mds
10:56:22 fsgetpath /Volumes/4 TB Utility 0.000011 mds
10:56:22 stat64 /Volumes/4 TB Utility//Applications 0.000030 mds
10:58:15 getattrlist /Volumes/4 TB Utility 0.000010 mtmd
10:58:15 lstat64 /Volumes/4 TB Utility/.MobileBackups 0.000004 mtmd
10:58:15 getattrlist /Volumes/4 TB Utility 0.000010 mtmd
10:58:15 getattrlist /Volumes/4 TB Utility/.MobileBackups.trash 0.000005 mtmd
10:58:15 lstat64 /Volumes/4 TB Utility/.MobileBackups.trash 0.000003 mtmdIt looks like the following processes were causing disk accesses:
mds/Spotlight/mds_stores- related to Spotlight. Since I don't need to search data on this volume, I went into System Preferences > Spotlight > Privacy, and dragged the volume into the list of locations Spotlight will be prevented from searching.mtmd- related to Time Machine. It looks like, by default, Time Machine keeps some deleted files on the disk, just moved into a hidden.MobileBackupsdirectory, to speed up recovery or make it possible to keep snapshot data available even when you're disconnected from your Time Machine volume (see OS X's MobileBackups. What is It?). But you can runsudo tmutil disablelocalto disable this functionality.
See also this post, which I found when researching usage of fs_usage.
Someday, when all drives (even giant ones) are SSDs, this will be a non-issue. But I still have a bunch of 2, 3, and 4 TB spinning rust drives kicking around, and I like the energy (and noise) savings that comes from letting the drive spin down for inactivity.
Comments
Just applied your "System Preferences > Spotlight > Privacy" suggested fix to Spotlight (added "don't search these two partitions on HDD) to stop constant external USB HDD accessing for no apparent reason. Happy to say so far it seems to be working. THANK YOU!