Mac Pro 2009 boots up in Target Mode but not independently , shows question mark in folder

I bought a really nice 2009 Mac Pro. At first it had a 500g drive, which showed the ? mark in the folder starting independently (not Target Mode)The seller sent me a 160g drive, and at that point both drives were up and available.

I then started them up in Target mode attache to a Mini with Yosemite. No problem both showed up. Based upon that success, I bought two Apple 640's, again just fine in Target mode, HOWEVER, the 160g drive (the second one installed) is no longer recognized either in independent start, Target mode, and in Disk Utilities.

Honestly I have no huge problem being able to run only in Target mode, but would like to be able to run the Mac Pro/Snow Leopard if I want to.

Have a feeling this is something simple, but I seemingly have no way of getting back into the Mac Pro to change the startup disk. I tried the startup with the option key pressed , no go there.

Anybody have any ideas?