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Richard59 - see if there's anything in here that helps ==>>
FAQ - Device Integration
Two thoughts - maybe ASUS disabled MTP (Media Transport Protocol), nothing any box builder does to software ever surprises me. Are you using an external USB 2.0 hub attached to a USB 3.0 port - that setup recently caused me much exasperation in another context.
I don't use calibre device integration. so I wont try to help beyond that.
BR