I'm late to the party, but I think I see what went wrong: the location of the referenced masters relative to the root of the hard disk may be the same, but you now have them on a hard disk with a different unique ID (because you copied the works to a different computer) - and this unique ID is used by Aperture to locate the referenced masters.
I think I got that right?
I suspect you'll need to import them again, but I may be wrong...
Consolidate Master and Offline Solved!
I have had the same problem as you. Cant merge my iPhoto Masters into Aperture. Cant find the Master files ant they are Offline.
When i go to "Locate recerenced files" i couls se that my referensed files were Offline.
Then i understand why.
I had changed computer, and on my previous Mac i had a different Account name, also was the iPhoto library in another location.
So when i looked carefully at the File Path i realised that this is the problem.
THE PROBLEM WAS.
Aperture dont regognice the correct pathway to your iPhoto library. By some reason, Aperture can locate the pictures when you locking at them offline, but when you try to Consolidate Masters, Aperture only remember the location you used maybe 10 years ago when you imported your iPhoto library for the first time.
Go to your photo and select the photos that is offline.
Select File menu and choose "Locate referenced files"
Click at the file path, is it correct, look carefully?
At the bottom left site you se your HD, Desktop....
Klick all the way down to the selected picture.
Now the same picture shows up and the grey button "Reconnect All" becomes active.
Press the button and now the pictures you have selected is Online and can be Consolidatet.