Why does a 4 TB WD drive show only about 3.63 TB in Windows?

Applies to

WD hard drives and external storage. My Book Duo RAID allocation and NAS volumes have additional usable-capacity rules; compare the physical disk separately from its formatted volume.

Work through these checks

1. Confirm the exact advertised capacity and inspect the disk's total byte count. For a 4 TB device, the decimal starting point is approximately 4,000,000,000,000 bytes. Do not judge the hardware only by the rounded free-space number in a folder window.
2. Compare like units. Dividing that byte count by 1,099,511,627,776 gives approximately 3.638 binary tebibytes. Windows may label that value TB. The conversion itself does not mean that files or storage have disappeared.
3. Distinguish total physical capacity, formatted volume size and free capacity. Formatting structures, existing files, recovery partitions and other allocated volumes can reduce the space available for new files. Note which number the application is showing.
4. For a My Book Duo or NAS, confirm the actual RAID mode and volume allocation through its supported management tool. A mirrored pair reserves a second copy rather than presenting both disks as one full-capacity space. Do not change RAID mode to recover an expected label total.
5. If the total physical disk capacity is dramatically wrong, check the exact model and connection with our Support Specialist. A zero-byte device or a disk showing only a small fraction of its expected size is not explained by the normal decimal-to-binary difference.

Confirm the result

Verify that the reported total bytes are consistent with the model and that any partitions or RAID allocation are accounted for. For a healthy readable drive, confirm the separate backup before changing its layout.

When to contact us

Escalate unexplained large differences, unallocated space after an unexpected change, I/O errors or suspected incorrect product identity. Provide the model and a redacted storage-management view.