I have seen a few people getting the dreaded Unknown USB Device (Device descriptor Request Failed) error when connecting their Yaesu FT5 to their PC. I should start by saying the supplied USB cable (YAESU SBR-36-MU) is for FIRMWARE updates ONLY. You will still need the SCU-57 for programming the radio. The PL2303 driver is also for the SCU-57. If you want to program the radio without the SCU-57 you will need to use an SD card.

Firmware update steps:

1. Remove the battery from the radio.
2. Move the switch on the back of the radio to the "Update" position.
3. Plug the USB cable into the radio and your PC.
4. Install the drivers (they are on the Yaesu website).
5. Connect the power to the radio.
6. When you are in the firmware update mode you will see the update screen, open the firmware update software. You can ignore COM port and Baud rate settings — just run UPDATE.
7. Wait. The software will look like it is not doing anything at first but it is. Wait a good couple of minutes and you will see it start to update.
8. When it is done, you're finished.

The manual is as unhelpful as ever, but hopefully these steps help you get your FT5 updated. Good luck!