macos interaction with external USB drives

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External USB drives and USB keyboard dismount on macos (Sequoia 15.1.1) when using Syncthing 1.28.9.1.

I have been using Syncthing for over a year on a Parallels Ubuntu virtual machine. About 3 months ago my 4 macos external drives and my USB keyboard have started dismounting and/or becoming unavailable while updating Syncthing directories. I get macos finder warnings about drives being dismounted or powered off without being ejected. The keyboard just becomes unavailable.

Yesterday, I moved Syncthing to my macos, and disabled Syncthing on Parallels. The same problem appeared under macos. That’s the only thing that changed.

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Welcome here, @Harry !

When it comes to the his area, Syncthing is just an app which uses the filesystem, so it is super unlikely to be the cause of problems like this.

I bet that some piece of HW is bad. One or more drive enclosures or USB HW in the computer, I would guess.

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Agreed. @Harry I recommend that you back up the data in the USB drives without delay.

Next, disconnect all external storage and test. Assuming that your USB keyboard is directly connected (and not through a dongle or a hub), if the problem recurs, you have a problem with your Mac’s USB hardware. The converse is not true however —- if the problem does not recur, this does not rule out the USB hardware.

Connect one external hard drive directly (without an hub or dongle) and test. If the problem does not recur, add another until it does. If the problem occurs with all external hard drives, I would bet on a USB hardware issue in your Mac. If the problem recurs with a single USB drive, you have your answer.

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Unfortunately, I don’t think it’s that obvious. Aside from assorted USB devices connected through a USB hub, I have 3 USB hard drives, the keyboard and the hub plugged directly into my iMac. 2 of the hard drives are dedicated to Time Machine, one is unused and one is Storage, for external storage. Storage is used by several mac applications. I have taken the following steps to try to isolate the problem:

  1. Turn off Time Machine. Problem persists, but frequency is reduced.
  2. Turn off Time Machine, Parallels Linux, macos and Parallels Syncthing. Other applications still using Storage USB drive. No problems.
  3. Run macos Syncthing, keep time machine off, keep Parallels off, other applications still using Storage. Problem greatly increased.
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Syncthing is simply not on the list of suspects for these kinds of problems because:

  • It does not acces the HW.
  • It uses OS API:s for filesystem interaction.
  • Even if Syncthing was misbehaving, the OS is responsible for shielding the HW from misbehaving SW.
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Respectfully, I think you’re mistaken. @martinleben is correct on all counts.

Syncthing’s use of the filesystem watcher may be triggering this issue. Or Syncthing’s periodic full scans. You could turn one or both off for testing. But it also could be just a matter of frequency, duration, or some other normal use of the filesystem that triggers the issue.

Again, I strongly urge you to back up your external storage without delay.

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Besides the macOS version, it would be helpful to know:

  • Which model of iMac?
  • Is the USB hub self-powered or host-powered?
  • 2.5" or 3.5" USB hard drives? Capacities? (or even better, the model numbers)

Given the amount and types of USB hardware, if it’s not a hardware defect as @martinleben and @chaos already pointed out and/or a firmware issue, it could very well be a power draw issue.

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Thanks for your interest.

  • 2019 27" imac
  • USB hub is self powered, but while trying to debug problem I unplugged it and the problem was still present. NOTE: Drives and keyboard are directly connected to back of imac.
  • USB hard drives:
  1. Storage - 8 TB Seagate directly connected to back of iMac, contains data accessed by Syncthing
  2. Time Machine - 8 TB Seagate directly connected to back of iMac
  3. Time Machine 2 - 4 TB WD My Book directly connected to back of imac.

Time machine is currently turned off. Both Time Machine drives have been unplugged and problem reoccured.

2 USB keyboards tried - Apple keyboard and MacAlly Slim, both were plugged into back of iMac during trial. Both stopped communicating with imac.

I’m beginning to think the issue is with the imac’s USB subsystem.

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