External Hard Drive Keeps Disconnecting on Mac: Fixes

If your external hard drive keeps disconnecting from your Mac, the cause is almost always the cable, the port or hub, or not enough power. Test those first, in that order. Only then look at sleep settings and the drive itself.

A drive that drops out while you are using it is different from one that won’t eject. You get no warning and no chance to save, just the “Disk Not Ejected Properly” notification afterward.

Test in this order

Change one thing at a time and use the drive for a few days. Whichever change stops the dropouts is your answer.

1. Swap the cable. This is the most common cause. A cable can charge a phone and still fail under the steady data load of a drive, especially near the connectors where it bends. Use the cable that came with the drive, or a short, known-good data cable. Throw the old one out if the problem stops.
2. Use a different port. Try another port on the Mac. On a laptop, ports that get flexed or bumped can lose contact.
3. Remove the hub or dock. Plug the drive straight into the Mac. If the dropouts stop, the hub is the problem. Cheap hubs can’t always sustain throughput or power for a drive plus other devices, and a dock can need a firmware update from its maker.
4. Check power. A bus-powered 2.5-inch hard drive draws all its power from the port. A hub, a long cable or a busy Thunderbolt chain can leave it short, and it drops out when it starts a heavy read or write. If the drive has a power adapter, use it.

Check the sleep settings

When your Mac sleeps, it can disconnect external drives, and the drive does not always come back cleanly on wake. This is the cause when the drive vanishes after a Mac has been idle, not at random moments.

Open System Settings > Battery (or Energy on a desktop Mac), look at Options, and turn off Put hard disks to sleep when possible. You can confirm what the Mac is set to in Terminal:

pmset -g | grep disksleep

A value of 0 means disk sleep is off. The default is a number of minutes.

Look at the logs

To see when the volume actually went away, run this in Terminal right after a dropout:

log show --last 1h --predicate 'process == "diskarbitrationd"' --style compact

diskarbitrationd is the macOS process that mounts and unmounts disks, so its entries show when the volume appeared and disappeared. If the timestamps match the moment your Mac went to sleep, it is a sleep setting. If they fall at random times, look at the cable, hub and power.

To check what the Mac currently sees on its USB ports, run system_profiler SPUSBHostDataType (on older macOS versions, system_profiler SPUSBDataType). If your drive is missing from the list while it is plugged in, the Mac isn’t detecting it at all, which points to the cable, port or the drive’s own power.

A failed filesystem check can look like a dropout

If a drive was not ejected properly last time, macOS may check it the next time you connect it. While that check runs, the drive can be grayed out in Disk Utility or slow to appear. From your side, it looks like the drive vanished.

Open Disk Utility, select the drive, and run First Aid. If the check keeps finding problems every time you connect, the cause may be repeated bad disconnections, so fix the physical problem first.

When the drive itself is failing

A drive on its way out can disconnect more and more often. In Disk Utility, select the drive and look at S.M.A.R.T. Status in the info panel at the bottom of the window. It reads Verified when the drive reports no problem and Failing when it does. Not every external enclosure passes SMART data to the Mac, so a missing status is not a clean bill of health.

If the dropouts continue with a new cable, a direct connection and sleep turned off, test the drive on a different computer. If it disconnects there too, copy off your files now and replace the drive.

While you are sorting it out

Eject normally whenever you can, even on a drive that is acting up, because every bad disconnect adds filesystem risk. When a normal eject fails, how to eject an external hard drive on Mac walks through what to try. The cause behind each Disk Not Ejected Properly warning is in fix Disk Not Ejected Properly.

Ejecta helps with the eject side: it shows which process is holding a drive and can retry the eject for supported apps. It does nothing for a flaky cable or a dropout, so fix those first.