Hi tain,
Thanks for the quick response!
Windows identifies the drives and they work fine, but the flashing software is not run from Windows - it's a self booting DOS-like program.
I tried this procedure 3 times, and all 3 attempts failed. To be fair, I couldn't find anywhere any mention of someone updating external Seagate drives via their firmware update tool. The flash util README doesn't mention USB drives either (just (S)ATA).
Thanks again.