You may need to trim the values to ensure that there are no trailing spaces causing a problem.

To check for a read failure you need to test the error values, something like:
Code:
$ValueX=ReadProfileString("X:\test.ini","Test","Test")
If @ERROR
	"Cannot check - failed to read value from X:" ?
	"Error was: "+@ERROR+",  "+@SERROR ?
Else
	$ValueD=ReadProfileString("D:\test.ini","Test","Test")
	If @ERROR
		"Cannot check - failed to read value from D:" ?
		"Error was: "+@ERROR+",  "+@SERROR ?
	Else
		If $ValueX <> $ValueY
			"Values do not match" ?
		EndIf
	EndIf
EndIf