Registered ECC won't work in an HP Z420.

No. The Z420’s memory controller expects unbuffered ECC (UDIMMs), not registered (RDIMMs). They look similar but the electrical signaling is different—the motherboard simply won’t train or boot with them.

The CPU itself (Xeon E5 v1/v2) can technically handle both, but HP locked the board to unbuffered only. You might get lucky with a specific firmware revision, but it’s not worth the headache. If you need more capacity than 64GB, you bought the wrong workstation. Stick with PC3-12800E or PC3-14900E unbuffered ECC sticks.

Don’t waste money on a gamble that probably won’t post.