when i first attempted to get my diecimila board running I got this error when i tried to upload the blink program.
the solution to my problem was that the user account on my macbook pro that i was logged-in under did not have administrator privileges. the build folder, i presume, is associated with the ftdi drivers that we have to install after installing the arduino sketch application. i am concluding that those drivers are only accessible by user accounts with administrator privileges.
indeed, when i logged-in under my administrator account i was immediately able to upload blink, and other sketches, and my board is now working as expected.
to those of you experiencing this build folder disappear problem, figure out how to give yourself administrator privileges and try again.
i hope this works for you.