In the DiskUtility go to "File>New>Blank Disk Image"
Save As: This is the name of the Disk Image File that you create ending with the dmg (or sparseimage) extension
FIle Selector Box: Select any location on any mounted drive where you want to store the Disk Image File.
Name: This is the name of the embedded Volume that shows up when you mount the disk image
Size: Select how much space you need
Format: select Mac OS Extended (Journal), this is the one that FCPx recognizes
Encryption: set to none unless you are working on sensitive CIA instructional videos "How to start War No4 and pretend it is for national security"
Partitions: I think you can leave that to "no partition map"
Image Format: select "sparse disk image" "or sparse bundle disk image". They are pretty much the same where the bundle chops up the data into small block. Both have the advantage that they expand meaning, even if you select a 2GB size, the disk image will only as big as the data you are storing into.
Once you created the Disk Image, it is just a file that you can move around (drives, Flash, Disc, etc). If you can see the file you just double click on it and it mounts like a mounted hard drive and voila it will show up in FCPx in the Event Library and Project Library.
This little trick enables you to work off volumes that FCPx can't see: TimeMachine Volumes, network drives, SAN, non HFS+ volumes.
You can find more details in my new manual:
"Final Cut Pro X - How it Works" and in the upcoming "Advanced" version
http://DingDingMusic.com/Manuals/
Edgar Rothermich