Matthew Campbell wrote: > I booted the Debian netinst disc and installed Linux on /dev/sdb1 as the > root partition. My computer is old. The system BIOS does not see this hard > drive, nor does Grub, but the Linux kernel does. I'm running the > 4.19.0-9-686-pae kernel, #1 SMP Debian 4.19.118-2 and Buster 10.4.0. > > The installation program tried to set up Grub on /dev/sda, but since Grub > cannot see /dev/sdb the system gets stuck in rescue mode. It sees two hard > drives hd0 and hd1, but says both have unknown filesystems. I had to > install Linux on a 32 GB USB flash drive just to get my computer to boot. > Now I can boot Windows again too. The flash drive is _really_ slow.
You should always install grub on the first bootable disk - even if it is the windows one. It took me a while to understand the problem. Try partitioning the new disk (sdb) into one smaller partition for boot (1GB) and the rest for root. Install grub on sda, because your bios is setup to boot from sda. Alternatively change bios to boot from sdb and install grub on sdb - this way you can go back to sda in the bios if you fail.