You can mix low voltage differential (LVD) drives without a lost in speed since the adapter will use the LVD protocol and access each drive at the highest speed it is capable. If you can mix single-end (SE) drives on a LVD SCSI chain, but the SCSI chain will drop to SE which is the common protocol and be limited to UltraWide (40MB/s) giving a lost in performance on the LVD drives.