If you plan to continue using the 6400 for a while, I'd suggest installing a USB 1.1 PCI card in one of the (2) PCI slots. You have to install a USB 1.1 card and not a USB 2.0 card. While USB 2.0 cards are typically backward-compatible to the slower data transfers of USB 1.1 on pre-OS 10.2.8 Macs, the Performa/Power Mac 6360, 5400, 6400, 5500, and 6500 models can't use USB 2.0 cards because they're multi-bus cards. Each port on a USB 2.0 card is on a separate bus, but these multiple-bus USB 2.0 cards confuse the PCI controller in the Macs that I've listed. Either OS 8.6 or 9.1 has support for USB devices, although the USB support files included with OS 9.1 (available in the OS 9.1 Update) can be used with OS 8.6 for greater compatibility. If you don't already have one, you could check eBay for a used USB 1.1 card. Once it's installed and the USB support drivers are loaded, you can use flash memory drives to quickly transfer files between the 6400 and your PC, because the 6400 can read/write to PC-formatted flash drives.