I cannot format my pen drive as there is some virus in it

VIRUS OR TROJAN INFECTIONS:

Virus or trojans can run some process that keeps running in background. This not only harms the computer, but also prevents the pen drive from formatting as it shows that the drive is in use. Remove and re-insert thepen drive, don't double click on drive and cancel autorun operations if it starts automatically. Run a virus scan on pen drive, this will help fix the issue. Now format the pen drive.

CORRUPTED FLASH CHIPS ON PEN DRIVE:

If the pen drive is not detected at all, or shows a less capacity, then there are high chances that its a chip corruption. Flash chips are the actual hardware components that save the data. A physical or electrical or electromagnetic damage to these chips make the pen drive faulty. This can happen because of voltage fluctuations and faulty USB ports. In such case, get the flash drive replaced if its under warranty as it can not be fixed otherwise.

FAKE PEN DRIVES:

Sounds a bit un-believable but true. There are a lot of pen drives in the market which are of very cheap quality which are fake copies of some known brands. Beware of such pen-drives. Always purchase pen-drives or memory-cards from a reliable source with bill and warranty to avoid any mess later.