[1 Drive in pool is not getting any data]

I have one drive in my pool that won't get any data. 
What are the steps to begin to troubleshoot this?

[Priority and exclusions help]

Hello,

I am new to all this but I am finding scanner not wanting to function the way I would like.

1. I would like to be able to exclude drives from scanning. I have a Raidbox connect via ESATA that I never intend to add to any pool, so I would like to be able to exclude it from scanning,.

2. Secondly, setting up scan order. There are drives that I would like to scan first but I don't see any way to say scan this drive first then this drive and so on. And it could help in my question 1, where if I couldn't exclude, then at least push the raid to the bottom of the pile since it will take really long.