6TB green shows 1.5 TB in Windows 10 64bit its formatted in GPT, I runned WD DLG extended check but it remains 1.5 TB in disk man and 1.6TB in WD DLG

WD Internal Drives Desktop & Mobile Drives
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WD6TBgreen_1
WD6TBgreen_1751×597 19 KB
WD6TBgreen_3
WD6TBgreen_3664×529 76.5 KB

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Is it attached using a USB / SATA adapter? If so, you need to make sure your SATA dock can handle large drives. Quite a number of them cannot.