Aperture won't open library from external hard drive

"The library could not be opened because the file system of the library’s volume is unsupported."

Have you checked the filesystem on the drive with your Aperture library? An Aperture library needs to be on a volume with the file system "MacOS Extended (Journaled)". Previous versions of Aperture did not check the file system and allowed you to store the library on an unsuitable drive where the library could become damaged.

This support document describes the requirements for external drives: Use locally mounted Mac OS X Extended volumes for your Aperture library

If your drive has a different filesystem move your Aperture library temporarily to a different drive and reformat the drive (reformatting will erase all data. You will need to copy all files you want to keep to a different drive before you do that).

Yes, if you have enough free storage on youri Mac to accomodate your Aperture library.