A processing unit for automated preparation of biological samples included in specimen tubes is described herein. The processing unit includes a removable sample preparation component configured to receive specimen tubes, read identifiers associated with the biological samples contained in the specimen tubes, mix reagents included in the specimen tubes, and transfer the specimen tubes to at least one of a centrifuge or a storage compartment. The processing unit further includes a centrifuge loader for automatic loading and unloading of the specimen tubes onto a centrifuge. The automated centrifuge loader includes a magnetic brake that can stop the centrifuge rotor at a precise position to facilitate loading and unloading of a specimen tube. The centrifuge includes a swing bucket designed to pivot around an axis and slide vertically to enable moving the bucket to a position in which the swing bucket can engage with the port where the specimen tube resides.