The primary goal of this multi-center Phase II study is to determine the safety and feasibility of a conditioning regimen consisting of busulfan (Bu)/ fludarabine (Flu)/ anti-thymocyte globulin (ATG) in adult patients with severe SCD. A two-component design will be used for this study. The first component will be restricted to patients who have an HLA-identical sibling donor. Five patients will be transplanted during the first component of the study. If no more than 2 of the first 5 patients experience unacceptable toxicity, including death, within the first six months after transplantation, then the safety of the regimen will be considered promising in adult SCD patients.