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.