How to Reinitialize old hard drive

[Musashi]

I switched the 1 TB hard drive in my desk computer with a 4TB SSD the other day after a clone disk operation. That part works great.

Now: I'd like to re-insert the old hard drive as an extra storage unit. Problem is the OS info is still on the old hard drive, and when both are installed, it insists on booting from the old one even when I bring up the boot menu and tell it to use the SSD. Only way to have the SSD OS in control is to remove the hard drive, thus I can't re-initialize the hard drive while it's installed. It also can't be read by my SATA-USB cable, which says on it 'For use with 2.5" Notebook drives only'.

So what's the method to reinitialize the old hard drive to introduce as secondary storage?