The capacity label is only a starting point
A new USB stick can appear in Windows with a plausible name and capacity before it has stored anything successfully. The first check is therefore a real write-and-read exercise, performed while the device is empty. Keep the receipt and packaging until you have established that the stick behaves as expected.
Some difference between advertised capacity and the number displayed by the operating system is normal because units and reserved space differ. That alone does not prove a counterfeit. More concerning signs include files that vanish, become unreadable after additional data is copied, or return different contents when read back.
Use disposable test data
Copy several ordinary files whose originals remain safe elsewhere. Open the copies and compare their sizes. Include a larger file if that reflects the intended use. A single tiny document only checks a small part of the available storage, so it cannot establish that the full advertised space is usable.
For a fuller check, use a reputable capacity-testing utility from its original publisher. Read its instructions carefully: a test may fill the available space or erase the selected device. Use an empty stick and verify the target before starting. Allow the write and verification phases to finish; the verification is the part that checks whether data can actually be read back correctly.
If errors appear, stop using the device for important work. Repeating a small copy until it happens to succeed does not resolve unreliable storage. Record the result for the seller or manufacturer and keep your original files on known-good storage.
Speed and reliability are different questions
A slow stick is not necessarily fake. Small-file workloads, USB ports and the device's own flash memory can all affect transfer speed. Compare performance only after basic integrity is established, and use a consistent port and test set.
Even a stick that passes should not become the only home of valuable material. It is easy to lose and can fail later. Use it for transport or as one of several copies, eject it normally, and keep the trusted original until the receiving computer has opened the transferred files.
