Why my iMac can only recognise my My Book Studio II 2T as only one drive with only 1T?

WD External and Portable SSD External SSD for macOS
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Most likely, your drive is configure as RAID 1.

Data protection (RAID 1)

Set the system to data protection mode (also known as mirrored mode or RAID 1) and the capacity is divided in half. Half of the capacity is used to store the data and half is used for a duplicate copy.