Thank you for the answer.
UPDATE:
Meanwhile i tried some more tests with another application and it has been reported data loss after 14,3 Gigabytes so maybe I found the real capacity of the pendrive.
The writing speed detected is about 2,75Mb/s and the reading speed is about 8,87Mb/s so that's why my Linear Write + Verify has just reached the 20% after 12h.
Surely the pendrive is defective, but what can i do to fix things?