How to Use Old HD After Adding SSD

The only place I can copy files to on the internal HD is the Users folder.

That's the way I do it when booted from the external and when writing to the internal (I do not use FileVault).

If you want to write the whole internal device as a data disk, erase it as APFS (case insensitive) via Disk Utility > View > Show All Devices > Erase the topmost device. Otherwise there are several permission obstacles.

Richard Groff2 wrote:

I cloned my older Intel Mac mini to an SSD and use that as my startup volume. I thought I would be able to use the internal HD as some extra storage, but it won't let me delete, erase, or copy anything to it.

The system-volume is read-only but you can write to the old data-volume (if it is part of a FileVault you must give a password to decrypt it, though).

If you do not care about the old internal device's content and do not plan to boot from it, you could erase it as APFS data-only disk (but I'd create a bootable macOS installer USB thumb drive and archive the desired macOS installer .dmg before doing so, just in case). On an Intel Mac this is simple but silicon Macs need the internal disk for booting so do not erase the whole internal device there.