Let’s see how I can explain it to you in a simple way and without getting into technical details…
At the beginning of time and shortly after God said let there be light… the guys who developed umbrelos did it thinking of a device like a Raspberry PI4 that would use an SD and a disk (HDD, SSD, etc.) Therefore and up to now, the code is programmed to detect an SD and a disk (HDD, SSD) and install and work like that.
But everything has continued to evolve and more powerful, efficient devices have arrived that do not necessarily need an SD.
So, I would like to think, the guys from umbrelos decided on the easy way: if it does not detect an SD, it takes the disk found (HDD, SSD, etc.) and installs and works on that disk in the same way as when there was an SD.
It does not matter if you have another disk (HDD, SSD, etc.) added and present. The umbrelos installer will detect only one and will mount on it.
Now, there is a solution to this: the easy and convenient one, the not so convenient one, which involves learning and being more free.
The easy and convenient one: wait for the next version of umbrellos, currently in Beta. The guys at umbrelos have promised that in this new version it will already be possible to add disks natively. Of course, you have to wait for them to release the version and for it to work as they promise.
The not so convenient one, which involves learning and being more free: HOW-TO Using 2 SDDs in umbrellos (in a simple and practical way)
From here, you choose your path.