I'm going to trust you tried a fresh install of the OS on each of the hard drives. I tried scouring lists of known error codes for you and I can't seem to find your code exactly. But some codes are unique to certain firmware updates so your code could be unique to 3.15. Also, it seems any code that begins with "SU" seems to be related to software update issues. I couldn't find anything on 30461 but seems like a lot of people are pulling a code 30641 so the two codes may be similar. I included links below on error information and how people fixed it. The YouTube video is helpful. http://community.us.playstation.com/t5/Consoles-Peripherals/Update-error-code-SU-30641-4/m-p/42454073#M27712https://www.youtube.com/watch?v=ap9M9kfueNE
Other thoughts why the firmware isn't updating
1) the one you downloaded on your thumb drive may be corrupted, thus corrupting all 3 drives you tried. Delete it from your thumb drive and redownload it and when you install, make sure that's the only thing on the drive. Other files may confuse the console.
2) thumb drive may not be formatted correctly. It needs to be FAT32 according to the YouTube video. Many drives now are formatted NTFS, and I believe thumb drives follow that suit now. FAT32 is an older format that isn't used as much anymore.