A short guide on how to partition and install SATA>NVME would be helpful.
Does the root fs reside on the NVME drive?
No special instructions necessary, just set up your bootstrap partition on APM like usual for GRUB (or yaboot, but yaboot is dead), plus a partition for /boot (doesn't necessarily need to be on APM, but it needs to be on an openfirmware visible drive, which is most likely to be the same one as the one with bootstrap, i.e. with APM); then / is on the NVMe, which can use any partition table that linux can see (i.e. can be MBR, GPT, APM or whatever you want, linux does not care and openfirmware only needs to see /boot so that the bootloader can see it)
then openfirmware will look up GRUB on bootstrap, exec GRUB, GRUB will look up the /boot partition, load a kernel + initramfs from it, and once a kernel is running, you no longer need to care about apple partitioning or whether the drive is visible from OF
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A cheap adapter like this:
https://www.ebay.com/itm/Pcie-To-M2...X4-X8-X16-Nvme-Sata-Dual-Ssd-Pci/273964257638
is useful to have for this. It has two M.2 slots, one for the NVMe drive itself, and one for a SATA M.2 drive, which only uses the PCIe to draw power and the data goes through a normal SATA cable, you can see a connector for it at the end of the card. So you put a SATA M.2 SSD in one slot (for /boot + bootstrap and possibly OS X), the NVMe SSD in the other slot, connect the card with a SATA cable to your mainboard, and you can have both SATA and NVMe SSDs on a single card in a single slot that way. Plus it's cheap
