Check SCSI devices to make sure they are all set to different SCSI ID's. By default, the host adapter uses SCSI ID 7, so drives should use ID's 0 through 6, or 8 through 15. Check cable integrity. Check the cables for solid connections. Make sure they are screwed down.
Inspect cable ends for bent pins.
Verify the ATTO host adapter has its termination set properly. Instructions to do this can be found in the installation manual. If used, verify the external terminator is the correct type and does not have damaged pins. Excess termination frequently occurs when these jumpers or resistors are not removed. This becomes more apparent as more drives are installed in the chain.