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I'm running Mojave 10.14.6 on mac mini late 2014.

It has internal hdd drive. I bought an external ssd to speed things up and run everything from it, and use internal hdd as a data storage\time machine

I installed OS on ssd and am running the mac from it, but I can't seem to erase the internal hdd drive. When trying to partition it in Disk utility I'm getting an error

The volume “osx” on disk1s1 couldn’t be unmounted because it is in use by process 82 (securityd)

After this error happened, disk utility shows that hdd has two partitions, 500gb each (that's how I set it up on 'partitioning' step), but I can see only the partition that was there before, no new partition that I added. Might sound confusing - please see the screenshots here.

Now, I'm very sure that I run OS from external SSD, so I don't understand why any process would use internal hdd and what should I do from now on.

So the question is, how do I use internal hdd as a data storage drive, and erase the OS from it?

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    put that internal drive in an external enclosure then boot and then connect that drive after.
    – Solar Mike
    Commented Apr 17, 2021 at 14:04
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    I'd suggest trying to unmount it first. You may need to work out what processes are hanging on it. Turn off Time Machine or any sharing that's already on.
    – benwiggy
    Commented Apr 17, 2021 at 18:07

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Thanks to a comment from @benwiggy I was able to solve my problem. Appears you need to explicitly unmount the disk before partitioning it. Not sure why it was mounted anyway. In any case, after unmounting with

> diskutil unmountDisk force /Volumes/my_disk_name

Partitioning went without error.

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