I've found a couple of other threads that relate to this, but neither replicates my particular circumstances.
I've got a 2010 Desktop Pro along with a two-month old Macbook Pro. Both were running Mavericks, now both run Yosemite.
I'm using InDesign CS5.5 on both - or at least trying to.
Everything works fine on the laptop; it did with Mavericks and it does with Yosemite. No problems at all.
But InDesign wouldn't launch on the desktop with Mavericks and it won't with Yosemite either.
Because I use InDesign very rarely (four times a year for two or three days apiece) I'm loathe to start forking out a regular (costly) subscription for InDesign CC.
Since I 'upgraded' to Mavericks and InDesign CS5.5 stopped working on the desktop, I've just been using the laptop. But with a 27-inch screen for the former and a 15-inch screen for the latter, it's clearly a ridiculous state of affairs.
Is the answer to just plug my monitor into my laptop? Maybe that's the simplest thing to do...