StarTech.com hard drive and flash drive duplicators perform a sector by sector copy, which means that the target drive must be the exact same size (down to the sector) or larger than the hard drive or flash drive that is being copied. During a sector by sector copy, each individual sector is copied over to the target drive, regardless of whether or not any data is present. A 500 GB hard drive with 50 GB of data on it will take the same amount of time to copy as a 500 GB hard drive with 450 GB of data on it.
If you have two hard drives or flash drives that are listed as the same size but are different makes or models, you need to check how large both drives are down to the sector to ensure that the target drive is the exact same size or larger.