Cannot boot into new drive's Recovery Mode, to erase old drive

Hi!

2012 Mac Mini with 1TB HDD, Sierra macOS installed, with Server.

Upgraded and installed a 1TB SSD, in addition to the HDD.

Can boot just fine from the SSD, restarting etc. each time, starts up on the SSD.

Set the Start Up disk inside System Preferences, and by holding down CMD and selecting it, after trying the startup manager.

Both SSD and HDD now have macOS Sierra with Server installed.

I now want to wipe the original HDD.

At first, was met with the cannot erase because cannot unmount issue.

So realised I needed to boot into Recovery Mode, on the SSD, and wipe the HDD from there.

Problem >> I cannot boot into Recovery Mode on the SSD. Each time, regardless of which drive I am in when I reboot, as soon as I hold down CMD+R to start recover, it chimes, and then fires up the HDD drive (I know given how slow the progress bar loads).

So since I'm in Recovery mode on the HDD, I cannot (understandably) erase it...

What do I do now? How am I going to wipe the HDD, if I cannot get into Recovery Mode on the SSD?

Suggestions?