USB Drives

[GeraldCC]

Hi Guys,
I had a usb drive (160GB) connected to a machine running KUbuntu 10.10!!
It was formatted as NTFS. I copied the data from it then went to format it to 
ext4.
This I could not do because it was mounted!!!! So I unmounted it and tried 
again, but this time the drive did not exist so I could not format it.
Could you please explain in simple language so a 74 YO can understand how to 
format a usn device?
Gerald