I'm trying to partition my external HD so that I can cap Time Machine backups at 100GB, but I keep getting an error when I try to partition. Currently I have two partitions (100GB for iTunes and 368ishGB for other files). I would like to split the larger partition into 100GB for Time Machine and 268GB for other files, but when I try to do this in Disk Utility I get an error ("Partition failed with the error:No space left on device"). There are 125+GB of free space on the partition I'm trying to split so I'm completely confused. Any ideas?