@Saywot Yes you have it right.
Rockstor’s preferred data arrangement is to use whole disk btrfs. That is each disk is used in it’s entirely, ie no partitions. Except for the system disk which is a normal partition arrangement. But yes currently Rockstor is a single system disk setup. But you can then add, by way of storage for your shares (subvolumes) collections of drives that form Pools (volumes in btrfs) that are then carved up virtually into subvolumes.
Btrfs is a volume managed as well as a filesystem (and in addition also has raid capability) and can use whole disks, ie /dev/sdd, rather than a partition on a disk, ie /dev/sdd1, this removes the need for the partitioning layer and any subsequent ‘value add’ volume managers such as LVM (Logical Volume Manager). Rockstor also does not support LVM installs.
So in short, install the Rockstor OS to a single device, then use the Rockstor Web-UI, not the installer, to manager the physically separate storage devices. This is required as btrfs is as yet not well supported by installers, with Rockstor’s capabilities, once installed, hopefully extending that capability so that one can easily form pools of disks that are in turn a collected resource with which to host subvolumes. Plus one can then alter the underlying structure of for instance the raid level whilst the subvolumes remain intack.
Give it a go and you may see the idea behind this. Single system disk with multiple Rockstor Web-UI initiated and managed whole disk data drives, collected into pools of chosen redundancy levels. Although best to stick with btrfs raid1 or btrfs raid10 for the time being as btrfs raid5/6, the parity raid levels, are not recomended for production use.
Also you don’t need to install the system to a USB key as the system disk, you can use a regular smaller hdd /ssd whatever. But if you do use a USB key make it a good / fast one or you will have problems as the quality of USB keys drops way lower than is generally available in the other formats.
Hope that helps to clarify things. And do take a look at the references I quoted for additional clarity, hopefully.