It's hanging on the library you copied over from the other computer? Is this library still on the other computer?
If so quit Aperture (If it won't quit Force quit it). Open the library on the other computer, conform that it opens OK.
Back on the second computer open Aperture on the copied library while holding down the option and command keys, run library repair. If the repair finishes ok (and it could take a while) Aperture will have the library open proceed to do what you started out doing.
If the repair fails copy a fresh version over from the other computer. It is possible something got messed up in the transfer.
thanks so much. i actually did a bit more research and found where you'd answered another person with a similar problem and repaired the permissions.
So far, that seems to have solved the problem.
may i ask what you think is the best option to combine the 2 libraries now that they are on one computer?
If both libraries are on one computer I would do the following:
Make a backup of both libraries. if the library you copied is still on the original system that can count as its backup.
Open library first aid on each library in turn and do a repair.
Open one of the libraries (either one) and do File->Import->Library and select the second library.
That should do it.