"Disk not ejected properly" message

Hoping someone has some insight into a weird behavior I cannot understand. 2015 MacBook Pro, OS X 12.0.1.

Starting a few weeks ago, I began seeing the “disk not ejected properly” message for two APFS volumes (partitions) on the same hard disk. But the volumes are still mounted and never were ejected. A second hard disk does not do this. It happens many times a day, possibly when the system Sleeps but not in a way I can replicate.

I thought this started when I was messing with the Battery (formerly Energy Saver) pref and checked the “Put hard disks to sleep when possible” setting, but have since unchecked it (for both Battery and Power Adapter) and the problem continues. This disk has been attached to the Mac for a year and the behavior is new. One of the APFS volumes is my nightly SuperDuper backup.

Can’t figure it out. Closest I could find is this, from 6 years ago but the answers don’t seem to apply:


Consider cable problems:

  • Try removing and then reinserting the cable to the disk (at both ends of the cable).
  • Try swapping cables or using a spare cable.

If you’re seeing this message and the drive was not actually powered-off or disconnected, then it sounds like the drive itself is going off-line.

This could be indicative of a failure in the drive’s electronics.

It could also be overheating. I saw the same thing happening to my Time Machine volume (a Toshiba N300 internal drive I mounted in a USB enclosure) after a few months. I transplanted the drive to an enclosure with a fan and the problem went away.

My suggestion:

  • If this is a drive you assembled yourself, check the thermals. If there is no fan, consider replacing the enclosure with one that has a fan. If there is a fan, make sure it is still working (and if the enclosure has a power switch or speed control for the fan, make sure it is on and running at top speed).
  • If you purchased this as an external drive, or if your cooling solution hasn’t failed, then the drive may be failing. Consider replacement.

I might also investigate USB issues (or however the drive is attached), but since you have another drive that’s not showing symptoms, that is less likely. But you may still want to try swapping out a cable or USB hub to see if it helps.

Great replies. I hadn’t considered that it might be a hardware problem. (Duh!) Makes sense.

I just observed it happening! Was working away and the alert popped up. Looked at the desktop and the volumes were gone and as I was looking, they reappeared as the volumes remounted.

I’ll start swapping things to detective the cause, will report back. Thanks for the insight.

Closing the loop on this: It was the USB hub.

I swapped cables, no effect; opened disk enclosure (old school way to troubleshoot thermal issues), no effect; swapped out the USB 3 hub – and bingo, three days running without failure.

Thank you, folks!