What will happen if I reformat my hard disk?

Used disk utility to reformat a partition...

• and that worked just great...only it did something to the other partition which has caused it not to mount, to refer to itself as disk3s5.It refuses to mount and if I click "verify disk" it says the disk needs to be repaired and to click repair disk. When I do so I get "disk utility can't repair the disk".So what on Earth did disk utility do to the other partition which I did not ask it to touch?The original job was completed because the file system of the other partition was NTFS and it's now MacOS Extended.Any ideas?

• Answer:

  1) Get your data backed up 2) Reformat entire drive, consider laying down the zeros 3) If you have it, run DiskWarrior on it (this has saved me many times) 4) If problem persists replace the drive (and if this drive is 3 years old or more and you need it to be reliable, consider replacing it now just to be safe). As to what actually is going on? Too many variables to know. I doubt anyone--even a computer scientist--could tell you for sure.