Actually, just the regular logs should be fine. That error is coming from the service and not from the file system. It basically checks for an existing
PoolPart... folder on the disk and if it sees it, it will refuse to add it to the pool. That check is there as a fallback in order to prevent adding the same drive to 2 different (or even the same) pools.
I would make doubly sure that there is no PoolPart folder on that volume. Also, don't forget to "show all files" because the folder is hidden by default.