I need to understand why a disk will get put in a missing/foreign state and what steps I can take to get them back into the existing RAID configuration without data lose.

I've seen a system with SSD drives magically show 2 disk drives as missing / foreign.  When it gets into this state I can either Import / or Clear the foreign bit.  However when I clear the foreign bit it seems as if I have to trash my VD and create a new one which is not acceptible in my situation.  I'm wondering if importing is a better option and what the caveats maybe if I import.  I've also noticed within the RAID Controller settings an option to auto import a foreign configuration and wondering if this is something that I should just start to set as I've seen this happen on many of my servers.

I've really only seen this happen on my SSD's and wonder if it is the backplane connection for the SSD drives and the fact that the drives don't fill the trays properly for them seeding well in the back plane.

So what I need is  a good understanding of this foreign / missing state (how / why the disk controller puts a disk in this state)?

What the best was to get this foreign bit to go away preserving data?  (two options clear or import, it seems to my clearing it means I have to recreate my VD?).  Is importing the option and will this likely cause future problems show corruption of data in future?  

As well as anything else I should know about what one should do?  

Lets assume their are no bad drives, I just ship the system to another location and when the server is started up, and two Drives in my RAID 5 set are in this missing foreign state....I want to preserve the data.  What do I do?