I’m running out of disk space on MacBook so I’ve velcroed usb drives to the lid. It makes it heavier, but I’ve gone from 100 G to 460G.

[update]: 2 1/2 years later (Dec, 2009), I’m using the same MacBook Pro.  The primary drive is now a 128Gig solid state drive and the CD-Rom has been replaced with an internally mounted 320Gig HD.  My external, velcro mounted, drive is a 500 Gig HD dedicated to backups.