There are no "internal" hard drives in the ADAT-HD24. The drive bays on the front panel house drives up to 20 GB each which can be removed and/or replaced while the machine is powered up, as long as the LED indicator for the drive is OFF. Because the HD24 uses inexpensive, off-the-shelf IDE-format hard drives instead of more expensive SCSI drives as its recording medium, it's practical to use a drive as permanent storage for audio instead of downloading to tape or optical disc.