RecruitingDrug trialSecond or Greater Allogeneic Hematopoietic Stem Cell Transplant Using Reduced Intensity Conditioning (RIC)This is a treatment guideline for a second or greater allogeneic hematopoietic stem cell transplant (HSCT) using a reduced intensity conditioning (RIC) in patients with non-malignant or malignant diseases. This regimen, consisting of busulfan, fludarabine, and low dose total body irradiation (TBI), is designed to promote engraftment in patients who failed to achieve an acceptable level of donor-derived engraftment following a previous allogeneic HCT.