After re-installing OSX, AppleTV and iPhone don't recognize iTunes library anymore?

Borrow some friends computers and authorize them to your iTunes account. When you’ve used 5 authorizations you can go into itunes account management and clear everything. Then you should be able to re authorize your computer.

When you reinstall the OS you need to de authorize first as even though the computer us physically the same some of the unique identifiers come from the OS.

You should have saved the entire ~/Music/iTunes directory on your old install to an external drive, without modifying it. Not just ~/Music/iTunes/iTunes Music.

If you have the old iTunes folder to hand, here’s what to do:
1) Turn off your AppleTV, unplug your iPhone and quit iTunes;
2) Delete the iTunes folder inside the Music directory;
3) Copy in the old iTunes folder in it’s entirety in the place of that folder;
4) Restart iTunes, you should see your old library immediately;
5) Play a purchased DRM’d track, and enter your auth information;
6) Turn on your AppleTV and connect your iPhone, and allow them to sync.

Let me know, hope this works! (And that you properly backed up the folder before reinstalling! If you were using Time Machine, let me know and I’ll explain how to find it in there. Otherwise, you should have just copied it to an external drive as-is.)

(The above method will also keep all of your apps and iPhone settings; I’ve done this twice now.)

Oh and when I say “you should see your old library immediately”, I mean that there shouldn’t be a progress bar while it re-catalogues things. The file ~/Music/iTunes/iTunes Library.xml and the other un-extensioned one already contain all that data.

@richardhenry

Yeah, I thought about that. Unfortunately, I had the “let iTunes manage files….” option on, which is a bit of a clusterf…, because as you mentioned /iTunes Music was copied to the external drive, but the actual library file and iPhone apps remained on the internal drive. I thought if I tell iTunes to go external it would copy everything over there, not only the media files. Not very smart!

Anyways, you were dead on with the problem. Unfortunately, I’ll have to go through the notions now :(

Oh well, thanks to both of you…