Re: Booting an old PC to SSD that the BIOS cannot see

[Pascal Hambourg]

Le 29/12/2020 à 04:54, Robert Furber via Support requests for the GRand 
Unified Bootloader a écrit :
Instead of sending them to the landfill, old PCs can be upgraded 
dramatically by replacing or supplementing the HDD by a SSD ..specially, 
a NVMe SSD. In my case, the NVMe SSD is mounted on a PCIe adapter card.

The catch is the BIOS in older PCs cannot see or be aware of a NVMe SSD. 
They can only boot from a HDD. However, after booting, the NVMe SSD 
becomes visible and accessible.

Old BIOS/UEFI firmwares do not have NVMe drivers. The PCIe-NVMe adapter 
card may have an expansion ROM providing NVMe BIOS/UEFI drivers, like 
SCSI or RAID adapter cards. You can check with "lspci -v".

The challenge is to figure out when, during the Grub boot, does the NVMe 
SSD become accessible,

Never during GRUB runtime. The SSD becomes visible only after the OS 
kernel takes over GRUB and uses its own native NVMe driver.

and then figure how to reboot on it. For 
instance, would it be possible to set up a customized Grub on the HDD 
that would chain a second Grub on the SSD

No. GRUB relies either on platform (BIOS/UEFI) drivers or GRUB's own 
native drivers (not enabled by default). AFAIK GRUB does not have a 
native NVMe driver either (yet).

If the adapter card does not provide an expansion ROM for the platform 
firmware, your only option is to install /boot and GRUB (for BIOS boot) 
or the EFI partition (for EFI boot) on a drive that the BIOS can manage 
and boot from.