Sudo diskutil apfs deleteVolume <diskIndex> failing

Hi there, small issue here.

After installing nixos using the holochain get started guide which is pretty standard plus some minor extra steps, and then rsyncing my nix store to another drive, I cannot elegantly remove the old Nix Store volume

When I run:

sudo diskutil apfs deleteVolume <diskIndex>

I get this output:

Started APFS operation
Deleting APFS Volume from its APFS Container
The volume "Nix Store" on<diskIndex> couldn't be unmounted because it is in use by process 0 (kernel)
Error: -69888: Couldn't unmount disk

I have tried a few things :

  1. Restarting
  2. Checking for processes using it (none are)

The only way I can get rid of it is by forcing an unmount and then deleting the volume, which never feels good.

Can anyone suggest what might be preventing a graceful unmount ?