Am 01.03.22 um 05:45 schrieb Robert Moskowitz:

How can I get the uboot off an SD card and then put it on another? dd is fine, but how much do I move?

everything between sector #0 and the first sector of the first partition (mostly something between 2048 and 8192 for non-android firmware, fdisk -l /dev/mmcblk[01] will tell you how much).
Keep in mind that the MBR partition table resides at the end of the first sector, so you may want to skip this section when burning the firmware to a different card,
see here: https://github.com/umiddelb/z2d/blob/master/odroid-c2/ubuntu-base-00.sh#L10-L11 .

The whole 4MB infront of the first partition? Or is there some way to determine the actual uboot size and grab just that?

In the past, there was no common / canonical way, see here: https://github.com/umiddelb/armhf/wiki/Get-more-out-of-%22Das-U-Boot%22#disk-layout-and-partitioning

Cheers
Uli