Why my external hard drive has less space than it should?

If your external hard drive shows less space than actual, one likely reason is that one of its partitions is corrupted. When a partition becomes corrupted, Windows may fail to recognize it properly and report it as "RAW" or simply leave it unallocated. In either case, the corrupted partition becomes inaccessible in File Explorer, and its storage space appears to be missing or reduced from the total capacity.

Corruption can occur due to improper ejection, sudden power loss, malware, or file system errors. Instead of displaying the corrupted partition normally, Windows Disk Management may show it as unformatted or with no drive letter, effectively making it invisible to everyday users. This can lead to confusion, as the space physically exists on the drive but isn’t being counted toward usable storage in File Explorer.

To recover the missing space, you'll need to inspect the drive in Disk Management. If the corrupted partition contains important data, you should first attempt to recover the files using data recovery tools. After recovery, or if the data is not needed, you can delete the corrupted partition and create a new one to restore full usable space on the drive. 

This is very effective solution to fix the external hard drive showing wrong capacity or not showing the full capacity in Windows file explorer.