hard drive dropped out of pool after restart

[Bigbuster153]

As title states, sever was running fine. 3 hard drives were initially in the pool, after a restart I got a warning that the pool status is degraded. 

Upon checking, I noticed that only 2 drives were in the pool "sda" and "sdc" but "sdb" is unused despite being reported as healthy.

any explanation/fix? It doesn't seem to let me add my drive back.

thanks.