Churning ext. HDDs

You can also use the macOS command line utilities "fs_usage" and "lsof". I haven't used these too much, but you can monitor activity on the system. You will likely need to filter the output so only information for the specific volume or drive is displayed.

This example will filter activity related to a drive with a drive identifier of "disk3". You can get the drive identifier for a specific drive from Disk Utility (only filter for a general driveID without the partition/volume ID which would be the "s1", or "s2" part of "disk3s1" for example). You want the drive identifier for the drive or perhaps the Container depending on how the external drive is formatted. For an APFS volume a drive will show a Container which will be attached with another drive identifier. For example the physical drive my be "disk2" while the APFS Container will be attached as "disk3". A mounted volume will be shown with an identifier something like "disk3s2", where we only need the "disk3" part for the filter.

sudo  fs_usage  |  grep  -i  disk3

It takes a little time for the command to start displaying data and it will only display data when there is activity related to the item you are monitoring. You use Control + C to terminate the command and return to the command prompt.

The above command should work for what you want. The "lsof" command will only be useful if a file is open long enough to be notice when the command is scanning, but there is a delay between scans. The following command will run "lsof" in repeat mode at one second intervals and scanning my external drive with the mounted volume name of "MyExternalHD" which is mounted in the "Volumes" folder (use double quotes to enclose the complete path, but you can also just drag & drop the volume from the Finder onto the open Terminal window after the "+D " which will properly fill in the path for you -- do not use double quotes if auto-filling the path):

sudo  lsof  -r1  +D  "/Volumes/MyExternalHD"

This command will need to be terminated by pressing Control + C as well.

I think I have correctly provided examples. These commands should be able to provide you with information on what is accessing the drive/volume.

Hi exrobotguy,

It sounds like you might have the "put hard disks to sleep," option disabled on your Mac. Here's how to make sure that is turned on:

1. On your Mac, choose Apple menu  > System Settings, then click Battery  in the sidebar. (You may need to scroll down.)

2. Click Options on the right, then do any of the following: 

Put hard disks to sleep: Turn on “Put hard disks to sleep when possible.”

Update your system while it’s asleep: Click the pop-up menu next to Enable Power Nap, then choose an option.

You can find that full resource here: Set sleep and wake settings for your Mac - Apple Support

We hope this resolves things for you.

Take care.

I use iStats Menu by Bjango , it does show you disk activity per disk. I think there is a trial version, not sure if there was a free one. I just pay for it because I like monitoring my machine.

BUT I concur with your initial post, for 2 weeks, my external HD churned for no reason, yesterday it stopped. I didn't change anything, all the things above I had tried too. Who knows. The external drive is not on spotlight either, so I am not sure why it was spinning.

Hey there exrobotguy,

From your last message, it's unclear if the steps you indicated taking resolved the issue with your external hard drives.

If they did not, we wanted to confirm if you took the steps sent earlier under the section 'Specify sleep and wake settings for a Mac desktop computer'? Here's the link again just in case: Set sleep and wake settings for your Mac

If the issue still remains after taking those steps, we wanted to continue ruling out some other possible causes:

1. Sometimes unexpected behavior can be resolved with a restart. Check out the section 'Restart' here: Log out, sleep, wake, restart, or shut down your Mac
2. We see you have macOS Ventura installed, but the latest version is macOS Ventura 13.0.1. You can verify this by clicking on the Apple logo at the top left corner of the screen, then on ‘About This Mac’. This link will show how to first back up, then Update macOS on Mac.
3. If your hard drives did not continue to activate earlier, when did this start to first occur?

Cheers!