Not all Macs play well with "Cable Select" settings even though many people will say that they will IF the Mac is a newer machine. Older models, and the occasional new one too, refuse to accept CS bridge settings and will not allow the drive to mount until the setting is changed to Master or Slave. Additionally, some Macs will refuse to recognize a drive set to CS if there is a second drive also set to CS on the same bus ribbon. In that case, try setting the drives to Master and Slave and install them on the IDE bus ribbon with the Master drive closest to the mother board. Another stumbling block is whether the drives are attached to separate IDE bus ribbons and there is another device (CD, Zip, Imation, etc) on the ribbon with the drive. Frequently, the speed of the drive is reduced because of the slower device being attached to the ribbon. I always set up the drives on one ribbon and the other devices on their own ribbon to avoid possible slow-downs. HTH.