diskarbitrationd on Mac: the disk mounting daemon and why it gets busy

What diskarbitrationd does

diskarbitrationd is the referee for every storage device the Mac sees. When the kernel reports a new disk (a USB drive, an SD card, a Thunderbolt enclosure, an iPhone in disk mode, a network share being mapped, or a disk image you double-clicked), diskarbitrationd takes over. It asks any app that has registered an interest whether it objects, checks the file system, decides where to mount it (under /Volumes), and mounts it. It then broadcasts "disk appeared" so Finder puts it on the desktop, Time Machine notices its backup disk is back, Photos sees a camera card, and so on.

Ejecting runs the same choreography in reverse. diskarbitrationd asks every process with files open on the volume to let go, waits a moment, unmounts, and tells the system the disk is gone. When something will not let go, you get the "disk is in use" dialog; when the cable is pulled before it finishes, you get "Disk Not Ejected Properly." Our safe to unplug page is about that second message.

It also handles disk images: every .dmg you open is attached as a virtual disk and mounted through the same path. Installers that mount an image, copy an app, and unmount it generate a small burst of diskarbitrationd activity each time.

What normal looks like

diskarbitrationd's CPU is a count of mount and unmount events. On most Macs that is a handful a day, so the process spends its life at zero:

Memory is a few megabytes and does not grow. If you see diskarbitrationd with meaningful CPU and no external storage attached, look for a disk image or a virtual machine that is attaching and detaching volumes on a loop.

Why diskarbitrationd is using CPU right now

This is a quiet daemon. There is no well-known Apple Community thread about it eating CPU on its own, because it never does. When it is busy, something outside it is generating disk events in a loop:

1. A drive on a bad cable or port. The most common cause by far. The drive loses power or signal for a fraction of a second, the Mac sees it vanish and reappear, and diskarbitrationd mounts it again. You see the icon flicker on the desktop and a stream of "Disk Not Ejected Properly" notifications.
2. An underpowered USB hub. Bus-powered drives on a hub that cannot supply enough current drop out whenever another device draws power. Plugging the drive directly into the Mac is the test.
3. A spinning Time Machine disk waking and sleeping. Some external disks spin down aggressively and are rediscovered on every backup. Harmless, but it keeps diskarbitrationd and backupd busy more than they need to be.
4. A failing disk. A drive that is dying resets itself repeatedly. If First Aid reports errors, or the drive clicks, copy the data off now.
5. Disk images or virtual machines in a loop. A VM that mounts a shared folder on each boot, or a script that attaches and detaches images, produces constant events.
6. A network share that keeps reconnecting. A server volume on a flaky Wi-Fi connection mounts and unmounts along with the network.

What it is not: malware, or anything a cleaning utility can help with. Security researchers have written about diskarbitrationd because of its privileges, but that is about Apple's code, not something running on your Mac.

Is it safe to quit diskarbitrationd?

No, and it is one of the few daemons where a force quit can actually cost you something. If you kill it in the middle of a mount or eject, a volume can be left in an inconsistent state until the next restart. launchd relaunches it within seconds anyway, so you get the risk without any benefit. There is no setting to disable it, and macOS needs it for Finder, Disk Utility, Time Machine, Photos imports and every app that opens a disk image.

The right move is always on the hardware side: eject the drive from Finder, then fix whatever is making it disconnect. If a drive is already flapping too fast to eject cleanly, unplug it, wait ten seconds, and plug it into a different port on the Mac itself.

Type any process name above for the same verdict. diskarbitrationd is a "leave it, fix the drive" process.

What to do, in order

1. Watch the desktop for a minute

   If a drive icon appears and disappears, or the "Disk Not Ejected Properly" notice keeps coming, you have found the source. Note which drive it is.

2. Eject it and change the cable or port

   Eject the drive from Finder (drag it to the Trash or click the eject arrow in the sidebar). Swap the cable, use a different port, and avoid hubs and adapters for the test. Many flapping drives are cured by a shorter, better cable.

3. Plug it straight into the Mac

   If it was on a hub or dock, connect it directly. If it stays stable, the hub cannot power it; use a powered hub or the drive's own power adapter.

4. Run First Aid

   Open Disk Utility, select the drive, and click First Aid. If it reports errors it cannot repair, or the drive makes clicking sounds, copy your data off it today and retire it.

5. Check disk images and virtual machines

   With no external drive attached, open Finder and look in the sidebar under Locations for mounted images. Eject any you are not using. If you run a virtual machine, check its shared folder settings; a share that reconnects on a loop generates the same events.

6. Restart if the drive is stuck half mounted

   If a volume shows in Disk Utility but not in Finder, or Finder refuses to eject it with nothing open, a restart clears the stuck state. Unplug the drive first, restart, then plug it back in.

When to worry

diskarbitrationd itself is never the thing to worry about. The drive behind it might be. Here is how to read the pattern:

Because the events are short, it is easy to miss them in Activity Monitor and only see the fallout: a corrupt file, a backup that did not complete. Safe to Quit records when diskarbitrationd and backupd were busy, so a drive that flaps overnight is visible the next morning.