External HDDs run for hours even when ejected. How can I stop that?

Use Activity Monitor app to see what apps are using the highest percent of CPU capacity. Next note that ExFat format is not optimized for Mac file size. HDs formatted in APFS will speed data file because of smaller compressed file size. Have you ever tried to recover files that have accumulated over one year with Time Machine ??? I have not heard of any successful TM Recoveries. I personally just ** my COMPLETE User's Folder to a separate "MY BACKUP" folder located in the "Users" Directory. This method of ** and subsequent Recoveries provides 100% "Bullet Proof" data protection - up to the last ** you made. PS - if using this method of Backup ADD this suffix to each BackUp you make to preclude overwriting the original - "(NAME)_MONTH/DAY/YEAR".

Click on external HD icon then do a Control Eject to stop the excessive Read/Write iteration once you determine what app is causing the problem. Best way is to stop the problem's app while in the Activity Monitor app.

first-photoholic wrote:

I use a MacStudio M1 and store my data on external HDDs (ExFat). After saving, these HDDs often continue to run for over an hour. When I eject them, they sometimes remain active for several more hours. The situation is similar with an HDD for Time Machine. The behavior of the HDDs is the same across different brands and therefore probably not dependent on the firmware. They are excluded from Spotlight indexing. What is running in the background and how can I stop it? I am grateful for any tips.

1) What macOS version is the Mac Studio currently running..?

First thing to do, is update to Sequoia 15.7.3 or... upgrade Tahoe 26.

What's new in the updates for macOS Sequoia - Apple Support

What's new in the updates for macOS Tahoe - Apple Support

2) Are the external drives connected directly to the Mac Studio..?

If they are connected to a hub or dock, then all bets are off.

3) What are the Mac Studios Energy Saver Settings..?

Turn on "Put hard disk to sleep...." if available.

To see which apps are active, launch Terminal and issue the following for whichever volume is involved:

sudo  lsof  /Volumes/YourVolumeNameHere

ExFAT is portable, but otherwise not a great choice.

For process IDs:

sudo fuser /Volumes/YourVolumeNameHere

You will need to provide an admin password.

My above Reply has "**" inserted where ** (Back Up) should appear. "will speed data file because " should be will speed data file transfer because.