hdiutil create -format UDBZ -size 640m -srcfolder tmp/build-dmg \
		-volname git-annex -o tmp/git-annex.dmg
hdiutil: create failed - Resource busy

Sometimes it has failed this way for weeks until reboot. Sometimes a single
retry makes it work. Despite many reports of this problem, I have never
found for an explanation for it, other than <shrug> OSX.