How to use internal HDD as a file sharing drive if I boot from external SSD on mac mini?

Question

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?

Answer 1

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.