I believe, having your main root file system on SSD/HDD is possible.
First there is a u-boot boot loader install in the SD card before and outside the first partition (boot partition). This is started by the firmware. After that u-boot will search for a u-boot script in /boot. This script is then has the commands for locating and running the kernel, initrd and a device tree file.
- Just remove the SD card and have the extract contexts imaged to SSD/HDD and remove SD card. If the firmware has the ability to boot SATA disk, then everything should work as is. I doubt this will work.
- Another approach is same as above but keep the SD card but without the ‘boot’ and ‘root’ partitions. I believe the u-boot will search for boot parition in SD card (wont find it) and then use the boot partition on SATA disk (which I believe it is able to access).
- Simplest approach is to simply move the ‘root’ partition into SSD. The boot partition and u-boot boot loader will stay on the SD card.
Each approaches require understanding the the u-boot boot process and may require a change to u-boot boot script. Checkout https://linux-sunxi.org and Olimex forums for further information.