I tried to transfer photos from my 2011 MacBook Pro (with Snow Leopard so limited to Lightroom 3) to a friend's brand new 2016 iMac (with Lightroom 5 not 6) via a Seagate portable external drive which I use to back up the MacBook (with Carbon Copy Cloner). The new iMac and Lightroom 5 were "virgin", completely blank. The import went apparently successfully but I found all the develop data had been stripped out. I applied a "remove chromatic aberration" preset during import (I don't know why Adobe doesn't make that the default—can someone explain) but could that have had anything to do with it? Surely the develop data would be stored with the photos in the Seagate, right? Is there anyway to add the develop data or am I going to have to delete all those thousands of photos and start from scratch again (this time with no presets)? Any advice will be much appreciated.
LR stores all develop data in the catalog unless you have the option checked to "Auto Write Edits to XMP". Then it create a XMP sidecar file.
So if you can copy over the original LR 3 catalog and then open it with LR 5 that LR 3 catalog will be copied and that copy converted to a LR 5 catalog and all the edits will be there "IF" you can relink the images file.
In other words, Importing is not the way to do this. Copying the catalog file and then opening it in Lightroom 5 is the way to do this. NO IMPORTING.
Surely the develop data would be stored with the photos in the Seagate, right?
No. By default, that's not how Lightroom works.
I was using the wrong command, should have been "export/import *as catalog*" instead of just "export/import". All sorted now. But thanks for your replies.