I had some similar issues with disks not showing up in the list and had a mix of issues (migrating from another pooling software):
1. Most of the disks not showing up were MBR format, which for some reason didn't show up on DrivePool. Converted them to GPT by deleting the partition, converting to GPT, then creating a new partition. Those disks then showed up.
2. For some reason, one of my disks had no space allocated to the partition. Did the same as 1, but just needed to delete and recreate the partition (it was already GPT). That one then showed up.
The disks were mixed between an LSI card and USB controllers, but now after fixing them above, they all show up and are in DrivePool now without issue.
Edit: Scratch 1. I had a disk I had that was already GPT but was not showing up to add to the pool. Doing number 2 restored it, even though the partition was fine and I had been using the disk independently prior.