It can have up to 8 slots of RAM and supporting 8 x 8GB = 64GB max. You are saying your machine has 12GB, can you see which slots/banks it is using now ? Sounds like it has 4+4 and 2+2. You can replace 2+2 with 8+8 then you have 24GB which is double the existing RAM.
You can use many combinations to get to 64GB
>>>> I'm wondering if I can safely run 1866hz ddr3 ecc registered up to 128gb?
No, 64GB max, and it only runs DDR3 RAM up to 1600MHz. Please use manuals on the following link to work on your machine specially a guide called Maintenance and Service Guide:
Thanks for the response. It has 6x2gb ddr3 ecc currently. I was reading on some other forums that people had achieved 128gb of ram at 1866hz on their hpz420's. Am I limited to 64 gb because of my bios or motherboard/ processor? A lot of people say these workstations can be upgraded past the factory recommended specs.
Be alert for scammers posting fake support phone numbers and/or email addresses on the community. If you think you have received a fake HP Support message, please report it to us by clicking on "Flag Post".