The part that I can't figure out is how to copy not just the partition but also Grub from one disk to a smaller one, which has a very different partition table. I can't just copy the MBR over.

What would the best approach be?

Couple of comments:
- you should never "just copy the MBR over". Way too much potential for breaking things.
- "clone" implies a sector-by-sector copy. I try to restrict its use to forensic copies.

You could simply copy the data (rsync, cp, whatever you prefer) then chroot in and update grub. Clean and easy - also easy to repeat in need if you get it wrong.

fsarchiver will clone your partition copying only used data and can restore to a smaller partition as long as the data fits.

Be advised that it also copies the UUID.

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Some folks have luck shrinking the original partition to just below new disk and use a backup scheme. Clonezilla and others might work then on partition.

gparted of course will copy partition and you might be able to re-install grub to this new disk first or later.

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Do you want to copy a bootable drive with MBR partition table to a new drive with GPT partition table?

Both were GPT or so I thought. Now I'm not so sure anymore.

I copied the partition with xfs_copy, booted some way I forget now and reinstalled Grub.

But I had to convert the new disk to MBR because Grub flat out rejected the GPT table.

Whatever. It's kind of solved. I didn't do exactly what the thread's title says, but now it seems that is not really possible. Even if it is, it has to be done in two separate, unrelated stages.

As the "boot part" is not IN the partition, you either clone the complete drive (not just the partition) to an identically sized disk (or just a very little bit larger) or you will have to do it in two operations, the partition as an image or archive backup and the boot part separately. As the "whole drive" clone copies the partition table too, no matter if it's MBR or GPT, it is not copyable to a smaller drive, although you may get away with if it's just the itsy bitsy smaller (but don't forget to shrink the fs afterwards, otherwise usage may try to use blocks that just aren't there).
To a really smaller drive (on which even the partition image won't fit) you'll need something like a file-by-file backup.

PS: as both gpt as well as grub use "the blocks inbetween MBR and 1st partition" grub will have to be REinstalled when you go from classic MBR to a disk with GPT, as grub has to know that to avoid using that gpt part of the partition table. It has to write its data behind the gpt blocks.

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You don't have to use "that" grub. You can use grub from other bootable media. The "c" CLI mode has a configfile command to use the grub.cfg on another device.

GRUB> ls
GRUB> configfile (hd1,msdos2)/boot/grub/grub.cfg

I tend to go this route so I'm booted to a clone before I install grub FROM the clone. You can install it in a chroot to, but it depends on what you're more "comfortable" with. Granted that you should "update-grub" in the chroot of the clone to have a grub.cfg that works. And update the /etc/fstab so update-grub uses something useful.

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