OK, thanks.
Generally on initial setup you'll probably select the options that defaults to a SHR pool of both disks. If it's manual or after setup then you can use the Storage Pool page and Actions->Add Disk. Which from the log messages it seems that you've been doing something like this already.
If you haven't done Add Disk while selecting Storage Pool 1 then try it again. Then next things I'd try as a way to test:
- Remove the disk and re-insert it. The DS720+ supports hot-removal of disks so you don't have to power down.
- Attempt to create a second storage pool using disk 2.
- Run an extented SMART test on disk 2 via HDD/SSD's Health Info (it will take hours but recently this enabled me to re-use a disk that had been flagged as ages ago as Failing and then became Healthy, this was after a quick SMART test said it was Healthy).
RAID 1 has a slightly better performance, not personally experienced, but SHR has better growth/expandability using disks of different sizes.