On Windows 98, you need to install separate USB drivers for just about every device connected via USB. That's why the thumbdrive and the USB/IDE adapter would not work. Get the driver for a thumbdrive, install it via floppy, then you can use the thumbdrive to add drivers for other devices.
Win98 drivers for older thumbdrives can still be had from some manufacturer's websites. There are drivers available for some of the USB/IDE adapters, as well.