Why does a USB drive lose space?
The lost space on a USB drive often occurs after trying to burn a system to the USB drive. Some operating system (OS), such as Chrome OS and Linux, will change the partition system of your removable device, so there will be little free space on your USB drive. Using some third party software inappropriately or an unsuccessful formatting may also cause inaccessible portion on your USB drive, which is shown as unallocated space.
A method of getting back this unallocated space to use Disk Part which is a utility built into Windows within the command line.
Warning: DISKPART is dangerous – do not use if you don’t feel comfortable with it! Insert your USB disk, make sure it is recognized by Windows (with the lower capacity), and then do the following: