Removable hard disc dismounts and remounts multiple times a day/night.

interrobang wrote:

Mac OS Extended (Journaled).

It’s been in use for 5–8 years.

This is a new misbehavior.

I did in fact dismount it after posting the above but did not disconnect the USB cable.

When I woke the machine this morning, I find it has remounted on its own.

MacOS Extended should be fine. We have dozens of those used for a photography business for storage of photos taken at various events (weddings etc.) and they continue to work fine.

A drive with a mechanical spinning platter in use for 5-8 years is the most likely cause for this, it is probably wearing out mechanically. New behavior, maybe yes, but likely it is gradual degradation that reached the threshold where it starts to cause symptoms.

I would obtain a new drive, copy all files to the new drive, and then use it and see how it works.

A hardware problem in the iMac could also cause this but that is much rarer, I think.

If you are using a hub or dock, I have seen those fail this way also. Try connecting directly to the Mac and test.

I would also try a new cable and adaptor, if you are using one.

Is it a spinning hard drive that’s powered through the USB cable from the Mac? Being old it may be drawing too much current and the port is shutting down or the drive can’t spin up. I have seen many drives overcurrent a port on just about every computer. I haven’t actually seen it go on and off, though . It usually just won’t spin up and that’s it.

i too have this problem on my macmini7,1 running 10.13.6

hdd dis/remounts

never had problem w/ samehhd on my prev machine mbp5,4/10.13.6

moving hdd/ps/cable to my laptop mbpro11,5/12.7.6 no problem so looks like usb bus on mini is flaky-/

is this a known issue? are there any h/w diagnostics available?

Just wondering if you have formatted them for Apples APFS?

i just ran diagnostics

now wghat?