alternate device
Computing term for a backup or secondary device that takes over when the primary one fails or is unavailable.
The system automatically switches to the alternate device if the primary device fails.
予備装置 is a more general term for a spare or standby device, while 交替装置 specifically implies an alternate device that takes over operation.
Compound of 交替 (alternation, change) and 装置 (device, equipment).