OneCare wants to configure my external harddrive by converting the storage space on the device to use a more secure file system, NTFS.
Will my existing files on this device be changed or deleted if I proceed with this conversion?
If you're going to use onecare to convert the drive. It should not change or modify your files.
But to make sure no spoilers will happen. I would suggest you backup your files before converting.
My recommendation is to ignore OneCare's suggestion and leave the files system on the external drive as FAT32.
-steve