That did the trick.
In case anyone wants to dig deeper into the how’s and why’s…
When using SDK Manager, the primary boot partition is the first one created, see below:
Device Start End Sectors Size Type
/dev/mmcblk0p1 40 29360167 29360128 14G Microsoft basic data
/dev/mmcblk0p2 29360168 29491239 131072 64M Microsoft basic data
/dev/mmcblk0p3 29491240 29622311 131072 64M Microsoft basic data
/dev/mmcblk0p4 29622312 29623207 896 448K Microsoft basic data
/dev/mmcblk0p5 29623208 29624103 896 448K Microsoft basic data
/dev/mmcblk0p6 29624104 29753127 129024 63M Microsoft basic data
/dev/mmcblk0p7 29753128 29754151 1024 512K Microsoft basic data
/dev/mmcblk0p8 29754152 29754663 512 256K Microsoft basic data
/dev/mmcblk0p9 29754664 29755175 512 256K Microsoft basic data
/dev/mmcblk0p10 29755176 29959975 204800 100M Microsoft basic data
/dev/mmcblk0p11 29959976 243531742 213571767 101,9G Microsoft basic data
… which makes it annoyingly complicated to extend the first partition with the space of the last one (adress spaces are not continuous… not impossible, but quite the effort).
When writing the .img file, the partition order is different:
Device Start End Sectors Size Type
/dev/mmcblk0p1 610304 28432383 27822080 13,3G Linux filesystem
/dev/mmcblk0p2 2048 133119 131072 64M Linux filesystem
/dev/mmcblk0p3 133120 264191 131072 64M Linux filesystem
/dev/mmcblk0p4 264192 265087 896 448K Linux filesystem
/dev/mmcblk0p5 266240 267135 896 448K Linux filesystem
/dev/mmcblk0p6 268288 397311 129024 63M Linux filesystem
/dev/mmcblk0p7 397312 398335 1024 512K Linux filesystem
/dev/mmcblk0p8 399360 399871 512 256K Linux filesystem
/dev/mmcblk0p9 401408 401919 512 256K Linux filesystem
/dev/mmcblk0p10 403456 608255 204800 100M Linux filesystem
/dev/mmcblk0p11 608256 608291 36 18K Linux filesystem
… with a large amount of unpartition space right after the first partition (/dev/mmcblk0p1 in my case).
You can either extend the partition yourself (using gparted, fdisk, or parted under Linux, pick your poison) or if all goes well, first time Ubuntu booting will notice the unpartitioned space and lets you configure it.
If you used SDK manager and you don’t mind having two partitions, you can always turn the big partition into a stand-alone bootable partition (and then it shows up as a regular SD card, already mounted on your Jetson).