SSD is a digital form of non volatile flash memory which is small in size and big in speed. An SSD can transfer its data or can work in GB/s. It works without any mechanical energy thats why its small in size. An SSD can be permanently installed on a systems motherboard or can be connected via PCI-E. In laptops, small PCI-E slots also know as M-SATA are present to connect an SSD. In PC, it gets connected via the SATA and CPU power port. Here, the size of an SSD gets the same size of an HDD, i.e. 2.5".

HDD is an old data storage type. It uses a mechanical energy to rotate the platters to read and write in it. The I/O is connected with another motor and the motor is connected to a signal wire, which sends the signals from the user to the motor. The motor then moves according to the user to reach the particular location on the disk. The data gets transfered via SATA port. Before this SATA technology, PATA was available, which is a single wide cable, which transfers both power and data via a single port.

Why HDD is cheaper than an SSD?

As, an HDD is using a mechanical energy, and does not requires any kind of flash storage, but an SSD works in a flash storage which is very costly in manufacturing.