I think you will need grub to boot /dev/sda1, as that is your /boot partition.

The whole problem has been solved, though not the way I'd want it.

My old hard disk had an empty partition, so I installed Debian 64-bit on it. I didn't have a choice, I had work to do and the old installation wouldn't work.

Debian 64-bit updated Grub, which detected the old installation and enabled it... or rather tried to. The old installation begins to boot up, but then there are many errors and sometimes it hangs, sometimes it just reboots before it's finished booting.

So I gave up, went back to Debian 64, opened the old partition and began the full-length migration, which is now complete. I had to turn down a job because of that. :-\

Thank you for your attention.

The problem is that when you installed debian the first time you selected "Build drivers only for this machine which it did, Now if you want to run the old debian wheezy you would need to compile a new kernel base on the kernel of the new machine.
I always build mine to include all the drivers, meaning I can transplant it to any of my other machines.