Main board IDE interface
There is generally only one IDE interface on the motherboard, and one data cable can hold two hard disks.
SATA interface of motherboard
If the motherboard has SATA interface, only one data cable can be installed, but generally there will be more than two SATA interfaces on the motherboard.
Small computer system interface
Hard disks with SCSI interfaces are mainly used in the server market, and Fibre Channel is only used in high-end servers, which is expensive and not considered.
Usb interface
Each usb can be connected to an external hard disk.
Serial r232 interface
Serial port r232 (generally only one computer) can be connected to an external hard disk, but it needs to be connected to a converter.