Unless the computer BIOS supports "Boot from USB devices" you will not be able to boot from a USB flash drive, floppy disk or external enclosure USB hard disk.
There may be an upgrade for the BIOS for the machine which gives this feature, but I doubt it.
There is virtually no support for USB in Windows 95 , evem in 95c & 95d it was very basic.
Windows 98 USB support is poor. Win 98 SE better. USB support also really only came in with XP.
Your USB flash drives may also be USB 2, which Win 98 has no native support for USB 2. USB 2 devices may not work with the machine because of this.