How to temporarily disable boot from EMMC Zero3W

I have succeed in installing an OS on the EMMC by using SFTP with SSH while running on an SD card.
I am now able to boot from the EMMC. The problem I have is that I would like to try a different OS and need to boot from an SD again to put a new image on the EMMC. When I install an SD card, the Zero 3W still boots the EMMC.

How do I temporarily stop the Zero 3W from booting the EMMC?
Is there a way to prioritize the SD for booting consistantly?

Figured out I can overwrite the boot partition (mmcblk0p2) to get it to boot from the SD enough to go in and overwrite the first 5GB of the emmc which allows me to boot entirely from SD.

Just in case someone needs to know my butcher method for killing the emmc.

I would still like to know how to alter the “boot order” to prioritize the SD. It would seem logical to me as a corrupted emmc could be rewritten by booting to SD, but the reverse is not true at the emmc is not removable.

@Stephen is it realizable?

The zero 3W can be booted from emmc. (by booting into debian from SD card, then SSH to host pc where the unzipped .img can be loaded into “Downloads” folder of SD, then be dd copied to the emmc) remove SD, boot from emmc.

I suspect one could also boot zero 3w to SD, download the .img.xz directly, unxz the .img and then dd onto emmc all from within the zero 3w. I chose to SSH so my much faster host PC could do the download and decompression.

The zero 3W can have the boot from emmc broken by dd writing zero’s over the first 100Mb.

I have not had a chance to try to sort out going into uboot and altering the order to prioritize SD.

Do you need help getting the zero 3W to boot emmc?
Do you need help breaking the emmc boot?