External hard drives disconnecting during large transfers

I’m in the process of setting up a new Mac mini as a home server. It’s got four external USB hard drives attached to it. Two of these are in active use (one for media, one for shared files). The other two are clone backups of the first pair.

I started out with all four drives plugged into an Anker USB 3 hub, which in turn was plugged into one of the USB-A ports on the Mac mini. However, several times while I’ve been copying large amounts of data from one of these external drives to another I’ve had issues where the source drive has disconnected itself, stopping the transfer. I’ve run into this both while manually copying files in the finder and while doing initial clone backups using Carbon Copy Cloner. I’ll come back and find the file transfer has failed and I’ve got a notification that the drive was unplugged without being properly ejected (though by that time the drive has reconnected). I’ve had this issue with two different pairs of drives (both source and destination drives are different), so I don’t think it’s a hardware issue with the hard drives.

What has seemed to help is moving a drive from the USB hub to directly plugged into the Mac mini. Once I did that with the first pair I haven’t had any more issues (though now I’ve filled both of the mini’s USB-A ports). Now that this has popped up with the second pair of drives I’m planning to buy a USB-C to mini-USB-B cable to I can move another drive off the hub.

Before I do, I’m curious if anyone else has run into issues like this? Right now I’m leaning towards directly attaching both of the primary drives rather than running them through a hub, but I’m wondering if getting a different hub might be a solution that would leave more port space on the mini.

Can you chain one drive to the other to free up a Mac port?

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If the drives are obtaining their power via the Mini (through the hub), and running into some power-sharing contention, then perhaps a powered hub would address that issue.


Are these USB powered drives? I have had problems using a usb powered spinning drive as an “always-connected” drive. It would randomly self-eject. This would happen in the middle of transfers or even when they were not doing anything. It would happen no matter where it was plugged in (directly to computer or into a powered usb hub). I have not had this happen with connected external ssd drives.

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All of these drives have external power bricks (and the hub itself is powered) so I don’t think it’s a USB power issue.

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Have you tried another powered hub yet???

have you set hard drives to not spin down if possible in energy settings?

I’ve had the same issue in the past, and re-setting this helped me then.

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To follow up on this:

Eventually, the drives started occasionally spontaneously disconnecting even when they were idle (though this was much less common than when transferring a lot of data). I swapped over to another Anker USB 3 hub and they still dropped off occasionally.

In the end, I got a USB-C to mini-USB-B 3.0 cable and attached the second drive directly to the Mac mini. That seems to have cured the problem. The backup drives are still connected via the hub, but they are only mounted during backup and they don’t seem to have any issues.

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