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It's possible that that's all the drive will hold regardless of what its stated "capacity" is. It's practically impossible to use all of a drive's 'capacity'. Essentially every file on the drive will necessarily waste on average half of a sector. Depending on the OS and the file system, the amount of waste might be much larger.
Bad sectors and possibly other elements might also reduce physical capacity, though details about what the drive is would need to be known in order to determine if they would and how much.
It's also a little suspicious that the amount you say is used is right close to one-half of the stated capacity, and a little less. There are a few potential reasons that a physical drive's capacity might be twice the amount that's stored on it. We'd need to know more about what the drive is, what the enclosure is, how it's configured, what the OS is, what the file system is, and perhaps various other details.