How to create a sparse disk image for Time Machine backups
From blinkenwiki
First create the sparse image:
sudo hdiutil create -size 500g -type SPARSEBUNDLE -fs HFS+ -volname TigerCat /Volumes/Passport/TigerCat.sparsebundle
Next tell Time Machine to add the disk to its list of backups:
sudo tmutil setdestination -p /Volumes/TigerCat
(From Evernote)
