4-generally in one system we can connect four physical harddisks. as a primary master, primary slave, secondary master, secondary slave. in one system: either four primary partitions or 1 primary or 2 primary or 3 primary + 1 extended and all logical partitions will be create under extended partations. hardisk device recognized as follows primary master : /dev/hda primary slave : /dev/hdb secondary master : /dev/hdc secondary slave : /dev/hdd suppose you have only single harddisk and going to install linux, maximum you can create 4 primary partitions. if you create four primary partitions you cant create extended partitions that mean no logical partitions can create.