Multi-partition SSD plugged into RPi with umbrel

Hi, I would like to use my 4TB ssd with umbrel, but I have data on it that I don’t want to erase.
Can I create a partition dedicated to umbrel ?
If so, how to tell it that it should not erase the rest when plugging it in?

PS: bonus question: is there a way to get umbrel up and running without a 1 TB ssd?
I’d like to run at least a few tests on my CM4 with 32 GB eMMC. Thanks.