4e9fe054_Implementing_a_backup_strategy_able_1___BACKUP_DEVICE_OPTIONS_Target_backup_device
[Advantages] Easy Reasonably fast (6) if operating at USB 2.0 'High speed' rate Files not lost if master drive crashes or becomes unbootable Drive containing backup can be disconnected easily so that it is then completely isolated from possibility of infection by malware Drive can be stored in a safe location or off-site If the backup is a clone of the master drive or a bootable partition from the master drive, and the PC's BIOS supports booting from a USB device, it might be possible to boot the PC directly from the drive in the USB enclosure. (11) [Disadvantages] Cost of hardware lf the enclosure is fan-ventilated it could be a tad noisy (7) (but in my view, having a fan in the enclosure is essential if the enclosure is used for more than 30 minutes at a time) Slow if the drive is only working at the USB versions 1 or 1.1 rate (8) Slow if USB 2.0 sockets or cables not to USB 2.0 standard (8) Not all PCs are able to boot from a backup drive in a USB enclosure.(12) [Target backup device] 
Drive in an external enclosure that has a USB interface to the PC
Implementing a backup strategy
Target backup device