External HDD's are prone to being damaged more easily due to the nature of thier operating and being bounced around if not taken care of this causes damage to the platters and heads inside. Therefore it is alwys adviseable to back up onto CD as well if you cannot afford to lose data. Access times for HDD's is considerably quicke than CD but once burnt onto a CD-r it is permanent and as long as the CD's are kept in good condition you will always have the data available.