copy a hard drive

i want to copy a complete hard drive(master) to a second hard drive (slave)without loosing any data or o/s.this just to replace the master hard drive completly if there is any problem.
1 is it possible without any cost.
2 how do i do it?
3 will ibe able to copy everything from the first hard drive to the second hard drive while the first hard drive is running( copying to other drive)
this to save time if i am replaceing a hard drive.