I think this'll give you the same result with a lot less work:

declare @Foo table (drive char(1),mbfree int)

INSERT INTO @Foo EXEC master..xp_fixeddrives

IF @@rowcount = 26

SELECT '*' as drive

ELSE

SELECT TOP 1 drive FROM @Foo ORDER BY drive

Or to see all the details:

declare @Foo table (drive char(1),mbfree int)

INSERT INTO @Foo EXEC master..xp_fixeddrives

SELECT * FROM @Foo

Warning; xp_fixeddrives IS undocumented