Here's a script that will help you find the disk drive with the most available space.
create proc os_bigdrive @driveletter char(1) out ,@debug tinyint=0
create table #diskspace(drive char(1) null , space int null)
if @debug > 0 exec master..xp_fixeddrives
insert into #diskspace exec master..xp_fixeddrives
set rowcount 1
select @driveletter = drive from #diskspace order by space desc
set rowcount 0