In MacFileDirectoryTest there is an assert:
self assert: (MacFileDirectory isAbsolute: ':Desktop:test.st')
But ':Desktop:test.st' is not an absolute path to my knowledge.
I changed the method to say this: 

testMacIsAbsolute
	"(self selector: #testMacIsAbsolute) run"
	
	
	self deny: (MacFileDirectory isAbsolute: 'Volumes').
	self assert: (MacFileDirectory isAbsolute: 'Volumes:Data:Stef').
	self deny: (MacFileDirectory isAbsolute: ':Desktop:test.st')

Karl
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