The Librem 5 wiki has instructions on booting from an SD card for your convenience. Beware that if you put the same image into eMMC and SD, you’ll end up with partitions with duplicated UUIDs, which may cause unexpected behavior during mounting.
The last stable Crimson build for the Librem 5 can be found here in landing: https://arm01.puri.sm/job/Images/job/Image%20Builds%20landing/lastStableBuild/. The librem5r4.img.xz file is the one you want.
I’ve not tried this with a Crimson image so take these instructions with a grain of salt.
Edit: The instructions should be done on a separate host computer (not on the Librem 5, though it can be done).
- Typically the image will be compressed in an archive file to reduce the download size (such as .gz or .xz). Extract the image from its archive file to get the file with the file extension .img.
xz --decompress "$HOME/Downloads/librem5r4.img.xz - Write the image to your microSD card using your preferred method, for example
dd:sudo dd if=$HOME/Downloads/librem5r4.img of=/dev/<DEVICE> bs=1M status=progress conv=fsync - During boot, hold volume down.