accessing external DB from other computer?

You can store your Aperture library on a second internal drive or a directly connected external drive. The file system of the drive needs to be MacOS Extended (Journaled). So if you want to recycle drives you used with a Windows computer, move all data off the drive, and reformat the drive with Disk Utility.

See this support document: Use locally mounted Mac OS X Extended volumes for your Aperture library

If you want to access the drive from different user accounts set the "Ignore Ownership on this Volume" flag, see: iPhoto: Sharing libraries among multiple users (the tile says iPhoto, but this holds also for Aperture).

If you want to use a library, that is on an internal disk on another mac, you cannot use the library over a network connection. You need a wired connection. Boot the second Mac in target disk mode, see: OS X Lion: Transfer files between two computers using target disk mode

Target disk mode will mount the second computer as an external disk.

You don't have to move the pictures folder. You just have to move the Aperture Library (.aplibrary file).

Two Macs connected via Ethernet shouldn't be used. You can connect this way, but Aperture doesn't support drives accessed over a network, which is what this situation would be. Just get a good external drive (Thunderbolt is best; USB3 isn't bad) and put the Aperture library on that.

As William Lloyd said, Jon.

Just move the Aperture library to the drive in question and double click it to open it in Aperture, when you want to access it.

i am migrating from a 2006 mac pro running Lion to a 2012 running yosemite.

The Aperture version you are having on your Lion machine will be an older version. Once you open the Aperture library from your Yosemite Mac, you will no longer be able to open the library on your Lion Mac.