You'll have some work ahead of you to try to get 98FE to access a USB Mass Storage Device. Pretty much all manufacturers only produced drivers for 98SE. You could try the information here:
http://www.technical-assistance.co.uk/kb/usbmsd98.phpFor a backup of your system drive, as ranchhand has said, if the computer can boot from CD, it would be best to use Acronis or Ghost, their boot environment can store an image straight onto a USB Mass Storage Device.
If your hard drive is a Seagate or Maxtor, you can download and use the free Seagate DiskWizard software, which provides a feature-limited version of Acronis TrueImage.