For both arms, about 11 samples of blood (about 1-2 teaspoons each time) will be drawn for pharmacokinetic (PK) testing at time points before and after you receive your first dose of busulfan. For Group 1, this will be on Day -13. For Group 2, this will be on Days -20 and -13. The study staff will tell you the blood testing schedule. PK testing measures the amount of study drug in the body at different time points and will help the doctor decide your dose of busulfan for Days -6 through -3. If the doctor thinks it is needed, PK blood testing may also be done on Day -6 during your dose of busulfan.