There was an error opening the database for the library “/Volumes/Aperture Master/Unknown/Aperture Pictures/Aperture Library.aplibrary”. what do I do now?

Start Aperture while holding down the option and command keys. You'll get a Photo Library First Aid dialog as below.

Try Repair Database first. If that doesn't work, then redo the startup with option-command and try Rebuild Database. These repair processes can take a long time with a larger library.

If all that fails, you can try creating a new library from scratch and then try importing the old library into the new. This sometimes can fix problems that can't be repaired the First Aid way.

Also make sure the drive where the Aperture library is located is working properly. You can check it with Disk Utility app.

Did you move your library recently to that drive "Volumes/Aperture Master/" or has it always been there?

How is that drive formatted? And what is your Aperture version? Is it the most recent Aperture 3.4.5?

This error message is usually accompnied by a crashlog in the error message panel. Is there a crashlog?

Then copy the error message and post the log here.

Launch the "Console.app" from Applications > Utilities.

Then click the "Show Log List" button in the toolbar of the Console window.

In the section "User Diagnostic Reports" should be the Aperture logs, sorted by date, if there are any.

Select the newest and post the first 50 lines of the report.

Will it let me know when its done? when do I try to relaunch the Aperture?

When it is done, it will open Aperture automatically on the library that it just repaired. It ay take a while to finish, sometimes hours. Just let it run.