I’ll wait for someone more experienced to comment on the missing drive.
On another issue. What is the ‘cache’ drive listed? Is it L2ARC? Your specs only show 16GB of RAM and adding L2ARC starts at around 64GB of RAM. L2ARC eats into RAM that can be used for regular ARC below that and should only be added if necessary, according to the ARC stats. Please see the ZFS primer and hardware doc section for your TrueNAS version for more info.