thanks corky! since i'm such a novice, a little of that was over my head, but i love to learn new things!
Sorry if I gave too much detail. In a nutshell, the user library folder (which is hidden by default in Lion and Mountain Lion) simply holds settings for any given program including settings the user can change in the program. All of the files in that folder are recreated either automatically by the program when launched again, or by the user making setting changes in the program again. Leaving them in place for a re-install is fine as long as none are corrupted.
Well, i kinda jumped ahead, and hope i didn't make it worse. I already put the corrupt AP library in my trash and it's emptying right now. i didn't see the need to keep it, plus with as many files as it contained (over 300,000 as i said), I felt like i needed to get it off my hd. kind of like smelly old food! (bad analogy, i know!)
Not a problem since you have a known good backup library on the EHD.
So, that said, if i understand correctly, i will open Ap after, import a few images-from where? I won't have a library any more. Then if i works correctly, I would drag the good backup from my EHD, to the PIctures folder in the Finder window, and then import to Aperture from there? Just do "import", and then that's it, or is it more complicated than that?
Once you launch Aperture for the first time after re-install, import any pictures you have to test with. Take a couple pictures on your camera and then import those. Make sure you can import successfully and that you can add adjustments. Then export them to anywhere you want (Desktop, Pictures folder, or any other folder) to make sure the export function works correctly. If all is fine, quit Aperture and then delete that library (which should be quite small and quick to delete).
Connect the EHD and then drag your known good backup library to the Pictures folder and then double-click it to load it. You should then be back to square one before everything went sideways (assuming the backup is a managed library with the Master (Originals) in the library).
Note - if you were running a Referenced library (where the Masters (Originals) were located outside the library package in Finder folders) - then you will need to make sure the Masters (Originals) are located in their correct locations before launching the good library so the library will locate them automatically. I mention this because you said in your opening post that you were combining libraries and relocating Masters so I am not sure what type of library the known good backup library is.
FWIW, the default library type for Aperture is a Managed library where the imported images are stored inside the library package.
The easiest way is to open Aperture and then in the left pane make sure the 'Library' tab is selected which is technically called the 'Library Inspector' and lists your projects. In the upper section called Library select the 'Photos' item. This will show all images in your library in the main pane on right.
In the upper right corner of the application window, you should see a collection of icons with a section title called 'Browser'. Click the left-most button so you can view all thumbnails of your images in the main pane.
Below that same collection of icons with the Browser name, you should see the search field and an icon that looks like a dark rectangle with a magnifying glass in it's lower right corner. Click that to bring up the filter dialog.
In the filter dialog, click the 'Add Rule' button at upper right and select 'File Status'. This will add a File Status item at the bottom of the filter dialog, which should have the File Status check box marked and a drop-down menu to the right. Use the drop-down menu to select 'Referenced'. If all the photos disappear, you are good to go and have a fully managed library loaded. If some photos are still showing, then you need to use the 'File menu > Consolidate Originals for Library' command to bring those originals into the library. Once you have consolidated the originals, you will have a fully managed library which you can copy to and from any drive to backup or run at your command.
Note - consolidating means locating the originals via the Aperture dialog boxes and letting Aperture do the work. The most important thing with a database is to allow the program in control of said database (in this case Aperture) do all the work from within the program. This way the program knows where all the assets are and can allow access to them at will.
If you have referenced images and run into problems, post back and I or some of the other advanced helpers will try to sort any issues.