Small Raspberry Pi computers can store OS and other data on microSD cards and USB flash drives. Ordinary external hard drive can be quite expensive (more expensive than entire Raspberry Pi computer). Therefore, Western Digital brings a new very cheap hard drive to the market. It is called PiDrive (WD3140LMCW) and it is aimed especially for Pi users. PiDrive uses ordinary 7mm slim factor and USB interface.
Capacity 314 GB is much higher than capacity of microSD cards but it is still much less than capacity of ordinary 2.5" HDDs. On the other hand, the special price for PiDrive is $31.42 only and you can buy 5 of them. Regular price was set to $45.81. The hard drive was optimized for very low energy consumption and it is compatible with Raspberry Pi Model B+, Pi 2 Model B and Pi 3.