Is a person considered a partner in sin if they give a flash drive to their friends after taking a pledge from them not to use it to transfer forbidden things like songs and movies, and then some of them break their pledge and use it for that purpose?

If you predominantly believe that the person requesting the flash drive will not use it for unlawful purposes, there is no harm in giving it to him. You will not be a partner in sin even if he uses it for illicit acts internally, because you are only responsible for what is apparent to you. However, if you later come to know that he used it for unlawful purposes, then do not give it to him again. But if you predominantly believe that he will use it for unlawful purposes, and you give it to him, then you are a partner with him in sin, because you aided him in his action, as Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." You should be firm in upholding the truth, remind these people of the danger of unlawful acts and lying, and choose your friends carefully based on righteousness, piety, and love for the sake of Allah.