Second display and mirroring

I just attached an Apple Thunderbolt display to my iMac. My iMac has a custom background but the Thunderbolt display has the default Apple galaxy one. When I clicked on "mirror" in display settings in, my iMac adopted the Thunderbolt's default background rather than my iMac's custom background getting mirrored onto the second display. This would lead me to believe that my iMac is mirroring the Thunderbolt rather than the other way around. Also, when I use the hot corner that I had set up before on my iMac to put the display to sleep, they both go dark at the same time but when I wake them up, the Thunderbolt wakes a wee bit sooner than the iMac. Is all this normal? Or have things somehow gotten backwards?
Diane

 

I'm trying to mirror to diagnose a problem, so if there is something wrong in the way my displays are mirroring I would certainly appreciate some help as it will impact how I interprete any problems. Did I make my question too convoluted?
Diane

 

When mirror is not selected, my iMac has the white strip. That was why I was surprised when after I selected mirror that my iMac adopted the default galaxy background instead of the thunderbolt display adopting the iMac's background. Also, when mirror was selected, my iMac's color profile changed the to one the thunderbolt had instead of keeping its ColorMunki calibrated profile. This is all quite confusing. Also, now with mirroring, I have lost the sounds in Mail that used to happen when mail arrived or was sent. Most strange.