2020 iMac Time Machine backup disk repeatedly disconnecting

So, what could be the reasons that this is happening now with your external backup drive?

• A failing or loose USB, USB-C, or Thunderbolt cable.
• The external drive briefly losing power (especially if it's powered only through the USB connection).
• The backup drive beginning to develop hardware issues.
• A USB hub, adapter, or dock intermittently disconnecting the drive.
• macOS updates or temporary software glitches that interrupt communication with the drive.

To help prevent this from happening, I suggest that you consider the following:

• Verify that both ends of the cable are securely connected, and if possible, try a different cable.
• If the drive is connected through a hub or adapter, connect it directly to the iMac to see if the issue persists.
• Open Disk Utility, select the backup drive, and run First Aid to check the drive for any filesystem errors.
• Make sure macOS is fully up to date, as updates can sometimes resolve USB and external drive communication issues.
• If the issue continues, check the drive manufacturer's diagnostic utility (if available) or consider testing the drive on another Mac to determine whether the problem follows the drive.
• Finally, I suggest that you try using DriveDx to fully test this drive for any issues that could be causing this.

Adding to the excellent suggestions above — here are a few more things to check on your 2020 iMac:

1. Check for macOS power management issues:

- Open System Settings → Energy Saver

- Make sure "Put hard disks to sleep when possible" is UNCHECKED

- This setting can cause Time Machine drives to spin down and disconnect unexpectedly

2. Check USB/Thunderbolt port health:

- Try plugging the backup drive into a different USB/Thunderbolt port on your iMac

- If the issue stops, the original port may be failing

- Also try a known-good cable, as frayed cables are a very common culprit

3. Check disk health with Disk Utility:

- Open Finder → Applications → Utilities → Disk Utility

- Select your Time Machine drive and click First Aid

- If errors are found and can't be repaired, the drive may be failing

4. Review Console logs for clues:

- Open Console (Applications → Utilities → Console)

- Search for "disk" or "TimeMachine"

- Look for repeated error messages around the time of disconnections — these can point to whether it's a hardware or software cause

5. Reset SMC on the iMac:

- Shut down the iMac

- Unplug the power cord, wait 15 seconds, plug back in

- Wait 5 seconds, then press the power button

- The SMC controls USB power management and resetting it often fixes USB dropout issues

Since this started for no apparent reason, the SMC reset and Energy Saver settings are worth trying first. Hope it gets resolved!

Hi,

macOS frequently puts external drives to sleep, resulting in an improper ejection error.

You should uncheck the drives to sleep if they are possible button.

If you have not already done so, your first order of business should be to obtain at least one additional backup drive.

Among other reasons, hard disk drive failure often manifests in symptoms such as you describe. You never want to rely upon one and only one backup.

Is your Time Machine drive an HDD or an SDD?

If an HDD is it self powered?

How is it connected to your Mac?

(1) is completely unnecessary, wasteful, and will do nothing other than contribute to premature hard disk drive failure.