Assumptions:
A) Your current folder for your bitcoin data is here: "~/Library/Application Support/Bitcoin" or here "/Users/Library/Application Support/Bitcoin"
B) Your current folder for your bitcoin-qt application is here: /Applications/Bitcoin-qt.app
C) You have an external drive named "My Passport" and your Finder, Preferences settings are set to display Devices. Select Finder, Preferences and tick the box to the left of External disks.

To do this on a MAC running OSX 10.9 follow these instructions:
1)Open Finder.
2) In Finder, using the Finder menu bar, select: Go, Go To Folder... , and type the ~/Library and press RETURN. 3) In Finder, navigate to the "Application Support" folder. You will see the Bitcoin directory.
4) In Finder, select, or highlight, this folder, then select: Edit, Copy "Bitcoin" from menu bar.
5) In Finder, navigate to your external drive "My Passport".
6) In Finder, create a folder called "Applications" on your "My Passport" external drive (see step "C" above after connecting external drive, if necessary).
7) In Finder, navigate to the newly created "Applications" directory.
8 ) In Finder, select Edit, Paste Item to start copying the "Bitcoin" directory to your external drive's "/Volumes/My Passport/Applications/" folder.
9) Once this completes and both the "~/Library/Application Support/Bitcoin" and "/Volumes/My Passport/Applications/Bitcoin" folders are identical. Rename the Bitcoin directory in "~/Library/Application Support/Bitcoin" to "~/Library/Application Support/BitcoinOLD".
10) Launch Terminal from LaunchPad, Utilities or from Applications in Finder.
11) CAREFUL: At the Terminal prompt type: "cd ~/Library/Application\ Support" no quotes. Yes, that "\" slash is necessary.
12) Type: "ls -laf" no quotes, and insure that the Bitcoin directory is now called BitcoinOLD.
13) Type: "ln -s /Volumes/My\ Passport/Applications/Bitcoin/ ./Bitcoin" no quotes. this command actually creates a symbolic link to your external drive. Yes, that slash "\" just after the "My" is necessary to account for the space in "My Passport".
14) Launch Bitcoin-qt from Applications in Finder and let the "Reindexing blocks on disk.." process complete. It should not be downloading anything until this completes and then will download only those blocks since your last synchronize to the Bitcoin network.
15) Once this completes, delete the BitcoinOLD folder using finder. "~/Library/Application Support/BitcoinOLD".