I had to have the firmware on my USB drive too when upgrading my Slim's HDD recently. The guide is not wrong.|
However I would be interested to know if it lets you continue the upgrade with a newer firmware than was already on your PS3. It's entirely understandable that a user could have begun the process at an inopportune time and the currently available firmware is newer than is installed on the PS3.
To be honest, if the prompt screen explicitly asks for the 3.61 firmware, I'd put my old hard drive back in and update my PS3 if you can't source a 3.61 file.
2600, SMS, MD, GEN, 32X, MCD, Saturn, DC, NES, Famicom, SNES, SFC, N64, NGC, Wii, PS1, PS2, PS3, PS4*, Xbox.