I just bought a 1TB external driver, but after I copied 100 GB data to the driver, releaised that the file system is FAT32. Below is the steps to convert the disk to NTFS without formatting:

Run as administrator in Vista

run the command prompt as administrator is to enter “cmd” in Start Search and then use the keyboard combination Ctrl+Shift+ Enter.

then run convert f: /fs:ntfs, (f: is the driver name)

Aftere completing, it is NTFS now.