My whole Library is on the Spinning Rust part (a
mounted volume on C: (the SSD) . Just Use the
Create/Switch: Move the current library to new location that is a choice on the Library Icon.
AFAIK the DB is kept in RAM while using, so other than
transfer rate on load, there is no speed to be gained. RAM... Lots of fast RAM ... is the way to go

I bought a Refurbed Dell Optiplex i7 that had 16G RAM, 780M SSD and 2T HD and a fresh W10 Pro
When I decided to check on drive usage

I only had under 1T available.
The System had never been configured for the HD... At all. It was there, spinning, but had not even been enabled in the BIOS.
I guessed on how to set up a hybrid as I could not quickly find a 'proper' configuration tutorial