Hi
You can format it with, for example,
Parted Magic or Windows' Disk management.
If you want to do a low level format - use
DBAN. DBAN will write zeros to all areas of the drive. That process takes time - about 1 hour for 20GB.
Before you use DBAN or Parted Magic - disconnect the other hard drives. It's easy to make a mistake and wipe the wrong drive.