USB pen drive: Read/write error, dd fails

I have a 256 GB pen drive, bought in a store from a legitimate and well-known dealer.

Since yesterday, the drive doesn't get recognized anymore:

• Gparted displays Input/output error during read on /dev/sdc
• btrfs check /dev/sdc shows No valid Btrfs found on /dev/sdc Error: cannot open file system

• dd if=/dev/zero of=/dev/sdc bs=1M works for a while. But then, ctrl-c doesn't work. Upon pulling the pen drive, it finally stops and always shows something around 4 GB, for example 4565499904 bytes (4.6 GB, 4.3 GiB) copied, 18351.3 s, 249 kB/s - no matter how long I let dd run.

• fdisk /dev/sdc fails with fdisk: cannot open /dev/sdc: Input/output error

• badblocks -w /dev/sdc runs and runs and runs and runs... but when I stopped it (after a block count of about two billion) it said it has only written to about 1.7 billion blocks

Is there anything else I can try? For example, letting badblocks -w run until it stops on its own?