Types of external drives

The term "external drive" refers to the storage unit consisting of one or more hard drives in a separate casing. The casing and its electronic module is called "enclosure" or "box".

Simple devices - USB and Firewire enclosures (boxes).

  • Fairly simple - based on a single converter chip.
  • The filesystem type is determined by the host computer (to which the device is connected). So with Windows you get either FAT or NTFS, and with MacOS you get HFS.
  • Accessed in a manner similar to a local drive.
  • Service and maintenance functions (formatting and error-checking) are done by the host computer.
  • May either use a separate power supply, or get power from the PC over the data connection (typical case with 2.5" USB boxes).

Complex devices - NAS, Network Attached Storage.

  • Have its own hardware, like processor and memory, and runs its own operating system (typically Linux).
  • The filesystem type is determined by the operating system embedded into the device (typically Linux ext2 filesystem).
  • Accessed over the network, either wired or wireless. Certain models have a USB capability, but it still relies on complex internals of the device.
  • Maintenance functions are performed by the device itself.


There are some devices, like Western Digital "MyBook" which is a hybrid of the two approaches. It has its own RAID processing, but a filesystem type is determined by the host machine. The data would typically be recovered from this type of the device if you disassemble it, extract the drives, connect them separately to the desktop PC, and then perform a RAID recovery. Refer here for an almost textbook hybrid NAS recovery example.

Continue to Typical failure causes.