I’ve added 2 external USBs of 2TB each to my Proxmox server and created a Resource Pool called USB_HDD containing both. I created an Ubuntu VM, but I can’t allocate all 4TB to it in one go - it only allows me to add each one as a separate SCSI device. When I start to install the OS it only allows the install onto one of the 2TB devices. I though the point of pools was to make the actual disks transparent, and present the pool to the VM so it sees it as one lot of space. Am I doing something wrong, or do I have to have it as 2x2TB disks?
ZFS can absolutely combine both into a 4TB single volume. Well like 3.78TB but you get the picture.
Note that, just like any other method which combines disks in a “RAID0” way, if one drive fails you lose everything on both drives.
But yes as others have said, “Pools” in Proxmox (or, more accurately, “Resource Pools”) are not related to storage but to permissions. A pool of VMs, of network bridges, yeah even storage but they’re not used for creating storage, only controlling access to it in an environment with lots of users (like in an enterprise).
ZFS and Ceph also use the term “Pool” but in a different context. They’re talking about a pool of combined storage, which is what you are looking for.
Put each USB disk into a ZFS vdev and then combine the two vdevs into a pool. You can add more drives into the vdev later to create mirrors within the vdev and get a RAID10-like setup once you have the means.