I have an old (2012) Gen 3 i5 that supports SATA II only. It can boot from a USB device. I have found a PCIe adaptor card that will accept a 2.5" SSD (SATA III). However, you can't boot from the card because the card slot is PCIe 2.0. Can I just put the /boot partition on a USB stick & have this combo work? The computer has a conventional bios (non-EFI).

Normally 2,5" SATA 6.0 GB/s SSDs work on SATA 3.0 GB/s controllers. Enable AHCI in BIOS if it is available. Running the SSD on a SATA 3.0 GB/s controller should not be the bottleneck in a Gen3 Core i5 based system. Avoid USB (Useless Serial Bus) in your setup because it may cause many problems. In a Gen3 Core i5 based system, bootable USB may be limited to USB 2.0 controllers.

It may work, will have to run grub-install to make usb bootable to boot directly from usb, not just copy files from /boot to usb.

Thanks for the tips & for steering me away from a usb solution. I think you saved me some trouble. I was pleasantly surprised by the speed of your responses.

That's not the usual reason an add-in ATA host won't boot at attached disk. The BIOS treats such a card as a SCSI card. The card needs to have its own BIOS to be able to provide boot services. Many do not, or the BIOS doesn't treat the card as hosting a bootable device. You may need to enter the card's setup to enable booting from it.

For simply hosting boot files because a system disk isn't bootable you could attach any old ATA disk partitioned and setup specifically for the purpose, and run the OS from the SSD.

I have multiple old PCs with SATA1 and SATA2 onboard hosts connected to SSDs. They're all definitely faster than HDDs. Often the speed difference between a 3.0 (SATA1) disk and a 6.0 (SATA3) disk isn't seriously better on a SATA3 host. The capability is often limited by the disk more than the interface. Don't let the SATA2 interface's speed limitation dissuade you from upgrading to SSD.

USB ought to work. I used to use OpenSuse CD network boot to allow me to then boot to usb drive. The machine was so old it didn't allow usb boot. Once grub is loaded it doesn't care what it does.
There are still some more advanced boot cd/dvds that could be put on a usb. I used to use SuperGrub a long time ago. Might still work.

You can try to install a Puppy Linux like Fossapup or Bionicpup on a USB, then from the menu install Grub4dos onto the same USB, and hope Grub4dos will recognize the distro on your hard drive.

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