I found two different specifications:
https://www.intel.com/content/www/us/en/products/systems-devices/sku/h99572218/asus-zenbook-14x-oled... The first link is from Intel.com with the precise model: ux540zb. I think that the storage capacity is only limited to 512GB.
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Storage
Total Storage size
512 GB
Solid State Drive Capacity
512 GB
Storage Media Type
SSD
https://www.asus.com/Laptops/For-Home/Zenbook/Zenbook-14X-OLED-UX5400-12th-Gen-Intel/techspec/ From Asus.com link, it is very confusing to see the unclear information. Asus should precise the total storage capacity and the default storage, In my case (ux8402ze,) I had to ask for the information from Zentalk before upgrading my storage from 1TB to 2TB.
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512GB M.2 NVMe™ PCIe® 4.0 Performance SSD
1TB M.2 NVMe™ PCIe® 4.0 Performance SSD
1TB M.2 NVMe™ PCIe® 4.0 SSD
512GB M.2 NVMe™ PCIe® 4.0 SSD