Hard drive nonstop read/write

Ever since updating my iMac to Catalina, my hard drive is constantly being written to and read as evidenced by the noise, the lowdown and Activity Monitor. Occasionally, it does stop and works fine, but when it goes through this process it can take hours to resume operation.


Apps writing appear to be mds_stores, mdsync, kernel task for writing. For reading, it is Kernel_task, secd, launchd, and syncdefaultsd. All root tasks, and yes, it does continue even in a newly created account.


It is making the computer unusable a great amount of the time.

Earlier Mac models

Posted on Aug 11, 2023 07:53 AM

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13 replies

Aug 13, 2023 05:59 PM in response to Head Techie

27.48 GB available on a 1TB Hard Drive


There is the reason this is happening. A 1 TB hard drive should never have less than 150 GB free. Beyond that point swap files will start having a hard time finding where to go, applications will be addressing the the least fragmented parts of memory as best as they can, avoiding bad sectors, and as a drive ages, it gets worse. Get two external hard drives of at least 256 GB to free up your space so you aren't constantly running into the point of diminishing returns. 15% has been found to be an arbitrary value that consistently has proven true as far as storage where that point is reached.

Aug 13, 2023 05:55 PM in response to Head Techie

The MDS stuff is Spotlight, try rebuilding the Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode? Could take 10 minutes.


Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

How to rebuild the Spotlight index on your Mac

If searching your Mac doesn’t return expected results, rebuilding the Spotlight index might help.


  1. Choose Apple menu () > System Preferences, then click Spotlight.
  2. Click the Privacy tab.
  3. Drag the folder or disk that you want to index again to the list of locations that Spotlight is prevented from searching. Or click the Add (+) button and select the folder or disk to add.
  4. To add an item to the Privacy tab, you must have ownership permissions for that item. To learn about permissions, choose Help from the Finder menu bar, then search for “permissions.”
  5. From the same list of locations, select the folder or disk that you just added. Then click the Remove (–) button to remove it from the list.
  6. Quit System Preferences. Spotlight will reindex the contents of the folder or disk.


If still not working...


Manually Rebuilding Spotlight via Terminal

If the aforementioned Spotlight control panel approach doesn’t spur a reindexation of the drive, you may need to initiate it manually through the command line. Open Terminal and use the following command string to do so:


sudo mdutil -E /

This basically asks for temporary super user status, which is why Terminal may ask you for your password (it may not if you’ve used a sudo command recently or are already logged in as a super user or root. The command asks the unix tool mdutil to reindex the spotlight database for everything on the computer, including external drives, mounted disk images, etc.


sudo mdutil -i on /

Rebuilding a drive index can take a long time, so be prepared to wait whether you do it through the System Preference panel or the command line.

Aug 14, 2023 07:45 AM in response to BDAqua

It hasn't seemed to fix it. BTW, rebooting with shift key until login screen did not appear to boot me into safe mode for some reason.


After about 8 minutes, I'm seeing syncdefaultsd WRITING over 1GB worth of data, secd 362MB, Kernel_task 343MB.


Kernel task has READ 125MB with mdsynch 73.3MB. My hard drive is still clicking away with only Terminal open and now Safari.


iCloud only synchs keychain, notes, contacts, and Calendars. Turning wi-fi off makes no difference in the constant accessing of the HD.


Aug 14, 2023 06:37 AM in response to a brody

I've worked on Macs for decades in video production and never had this kind of nonstop use of the hard drive with zero applications running (I even use to sell Macs). I don't think you understand the severity of the problem. I suspect the solution lay elsewhere such as needing to rebuild the spotlight database, but if that doesn't work, I'll give what you say a try.

Aug 14, 2023 08:28 AM in response to a brody

Thanks. The HD seems to be fine according to the test although a bit low on the temperature issue. Yes, I have over 200GB of free space on the internal 1TB drive and it is the only drive typically hooked up (no ssd drive).


The Kernel_Task seems to be the main hog. At times the computer works just fine with just occasional hd access/clicking. Nice fast computer to be from 2013. Other times, when it is doing this accessing it bogs down the whole computer until it is finished we can be a half hour or more. I'll keep monitoring and see if the spotlight rebuild fixes things.


Thanks again for the suggestions.

Aug 14, 2023 08:49 AM in response to Head Techie

Keep in mind all automated processes will cause tasks to increase in use. This includes backup with Time Machine, iCloud, Microsoft OneDrive, open applications, Spotlight as you were looking, file sharing, screen sharing, web surfing where caches are created, temporary file management when autosave is on a word processing or spreadsheet document, etc...


If any folders are stored on iCloud, they will constantly be trying to sync up with the server any changes, and keeping a temporary file at the root level of the hard drive.

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Hard drive nonstop read/write

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