Hmmm . . .
I am trying to recall what I did when
Yosemite was released. I know I downloaded and installed it on my 2011 MBPro
AND made a bootable version of it on a flash drive.
I believe that I also installed
Yosemite on the iMAC from the
SAME flash drive.
I cannot say I did this for certain. However, if I had to download again from Apple to install
Yosemite on the iMAC, I would have also made a flash drive of the OS X as I did with the MBPro - as is my habit with each new OS X release. Since I only have one flash drive with
Yosemite, I figure I must have used it for both machines . . . otherwise I would have been compelled to make a second flash drive when I went to install
Yosemite on the iMAC.
Of course, I may have lost all my marbles.