Recently I encountered a situation where my usb shows only a very small amount in its capacity (roughly ~30KB !!!).

After researched, I found a solution and the steps can be achieved below:

Using Windows

To fix this issue, we are going to use Command Prompt on Windows

Open the Start Menu, search for Command Prompt, right-click on it when you see it in the results, and select Run as administrator.

If you are using Windows 10, You can open Powershell as well.

Once the Command Prompt is opened, type the following command and hit Enter. It’ll launch the command-line version of the Disk Utility program.

diskpart

Type in the following command to view all the disks attached to your computer. Your USB drive will be one of these disks.

list disk

To perform an operation on your drive, run the following command replacing N with the actual number for your USB drive.

select disk N

Use the following command to clean up your USB drive.

clean

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When the drive is cleaned, run the following command to create new partitions.

create partition primary

Finally, use the following command to format the drive to FAT32 format. (or to NTFS if you replace fs=fat32 to fs=ntfs)

format fs=fat32 quick

Once your drive is formatted, it’s recommended to unplug it and then plug it back into your computer.
You’ll then find that your computer allows you to use the full storage capacity of your drive.