I can’t answer the questions on Xubuntu 20.04, but if you first clone your SD card, or simply use a different one and don’t plug in your old SD card, you can then go back later and flash the QSPI part of the Nano again with the same JetPack 4.6, and that card (or clone of SD card) will work again.
What do you mean by flash QSPI part of the Nano.
And my last question is: I’ve already made a clone of my actual SD card on which jetpack 4.6 is installed, so if I install a new system on my nano and I want to go back on on my first system, you suggest me to “flash QSPI part of the Nano” (how do I do that), flash another SD card with the one I’ve made and so plug it in my nano and boot, if I have understood well, on my jetpack 4.6 ?
eMMC models of Jetsons have the equivalent of BIOS software in partitions, along with boot content (there is no actual BIOS hardware). Actual operating system content is the rootfs/APP partition. In SD card models that content still has to exist, but there is no eMMC, and so it is put int QSPI NOR memory on the module itself. This can be flashed separately or at the same time an SD card is flashed.
One normally thinks of changing a release of a Nano dev kit (SD card model, which implies it has QSPI memory) by updating the SD card since this is the actual Linux software. However, you can choose to leave the rootfs/APP/SD card alone, and flash only the module (QSPI flash). Necessary if the boot content and SD card content are too different in release.
There is no official Ubuntu 20.04 for a Nano. Someone else may have already worked on this, I’m not sure of what state it might be in or where to look at it other than what you’re already looking at. However, the boot content which leads up to that o/s for rootfs can be flashed separately and defaults might work with other Linux operating system flavors or releases. An example of flashing this on command line for a Nano (just boot content, not Linux itself): sudo ./flash.sh jetson-nano-qspi mmcblk1p1
Note that in the “Linux_for_Tegra/” directory there are several “.conf” files. When the flash target is “jetson-nano-qspi”, then it implies flash refers to “jetson-nano-qspi.conf”. This is a human-readable file and you can look through the chain of files defining that target. This will have an effect on things like device tree and bootloader configuration.