First of all, make sure your library has no problems before you try to move it.
Run the Aperture Library First Aid tools to repair it:
Once the NAS is mounted on your iMac and the iMac sees it as a disk, simply use the Finder to drag the Aperture library from the NAS to the folder on your iMAC where you want to install it.
Then double click the Aperture library in its new location to open it in Aperture.
You will have to make sure that the NAS is mounted before you launch Aperture.
For more details see this section in the Aperture User Manual:
Should Aperture report missing original files after moving the Library, use the "Locate Referenced Files" dialog to reconnect the Originals.
Are you sure, you want to leave your referenced images on the NAS? There have been quite a few posts recently reporting problems with referenced images on a NAS.
Thank you Léonie.
I will follow your recommendations for the library migration and report back. Thanks so much for your help!
At this stage I am aiming to keep all my images as referenced files on the NAS. Apple Supprt have advised me this should be quite OK - and I already have more images than will fit on the iMac. All our data for all our computers here at home use the NAS as the only storage. We are in the process of builing centralised off site archiving and backup, which is made a lot easier with all data being one centralised place - ie the NAS. It also means we can easily view slide shows on our big TV via DLNA just using the TV's remote control - using the same referenced image files (or exports if they have been adjusted with Aperture). I can also share photos easily straight from the NAS as my own private cloud with no cost or restrictions. (http://www.readynas.com/?p=5370). I do like the way Aperture does not tie one to the organisational strcuture of the referenced filing - so one is free to use all the advantages of Aperture's project, album, metadata systems ets. I am aiming to continue this way unless there is some change that causes me not to.
Once a few niggles about permissions across our home network were resolved, I have had no problems with both the library and referenced image files being on the NAS. The casual user would think the iMac was "it" for everything stand alone ... (unless the ethernet cable was puttled out of the back of the iMac!). Nonetheless I will take Apple Support's advice and place our ever growing single library file on the iMac following the 'precautionary principle'.
I will post again with my results soon.