I was not aware it supports GPT. I will test this.
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The MBR limit of 2T is true for typical/normal drives and enclosures which uses 512k logical sectors. Some enclosures+drives like your MYBOOK can use 4k and thus support the bigger cluster sizes which allow MBR support over 2T. But this...
The limit is that you have to use MBR and thus limited to 2.2TB TOTAL. You cannot use the extra space on the 3TB. To use over 2.2 you have to use GPT which is not supported by USBGX.