I'm completely confused:

I duplicated all partitions (yes, I kept types, size and position the same) from /dev/sdb to /dev/sda. (/dev/sdb is the original, working linux-only drive). Don't know if it's relevant, but the old is a SATA, the new a PATA.

I then went through the mystifying process of installing grub onto /dev/sda. That seemed to work, as specify that drive in the bios boot menu does startup linux.

But then, if sdb is left enabled (in bios), the machine does boot; all looks normal, but /dev/sdb1 is mounted on "/", and sda isn't even mounted.

I checked /etc/fstab (yes, the one in sda), and it referenced by UUID. So I tried to change that entry to use "/dev/sda" in place of the UUID, changing nothing else on said line. And it did exactly that: changed nothing. It still boots using /dev/sdb1 for "/".

If I disable the sdb drive in bios, the boot just hangs at"Loading, please wait".

What am I doing wrong?

chainload the other.

Grub on /sdb boots

press c

>root (hd0,0) (or try (hd1,0))
>chainloader +1
>boot

If it works make a suitable entry in menu.lstor grub.conf

chainload the other.

Grub on /sdb boots

press c

>root (hd0,0) (or try (hd1,0))
>chainloader +1
>boot

If it works make a suitable entry in menu.lst or grub.conf

Thanks for the tip.

I'll give that a try, but my goal is to get rid of the original drive, and also to understand why the new drive doesn't load (or somehow mounts the old drive).

Any other thoughts on why this might be happening?

How ? - precisely.Again how - precisely. Your supposition isn't supported by the following evidence.Looks like you haven't updated grub to use the correct disk. If it was me I'd pull the old disk out, and fix things from a liveCD. Tell us what you did, we can suggest fix(s).