Error copying Windows 10 files to USB

Still haven't figured out the problem. I did a fresh install of El Capitan, everything is up to date, no help.

I've got the ISO stored in the Documents folder. As noted above, the SHA1 checks out.

Screen shots of the errors:

This is the first one I get:

If I click Continue, it seems to format, but then when it starts copying Windows files, I get this:

Here's a couple shots with info about the USB drive I'm using:

Loner T, you out there? You're my only hope!

Well, I went out and purchased several different USB flash drives. None of them work. I get the same errors regardless of the brand.

I tried a Lexar USB 2.0 16 GB. I also tried a Gorilla USB 2.0 16 GB. I also tried a USB 3.0 flash drive.

None of them work.... I'm getting really frustrated...

Isn't there some way to create the USB drive manually? I'm able to download all the Windows Support files, drivers, etc. using BCA. But I'm not sure how to arrange everything on the USB.

Well I created a USB drive manually, by copying over everything in the ISO, and then copying over all the Windows Support files.

That gets me past the errors above, but then when it comes time to partition the hard drive, I get this error:

Crap...

This is the iMac with the 1 TB SSD drive.

So I tried something else. My iMac has two hard drives: the 1 TB SSD, and a second, 6 TB SATA hard drive.

So I reinstalled El Capitan on the SATA hard drive, and reformatted the 1 TB SSD. Then I tried to install Windows on the 1 TB SSD drive. Again, I get errors at the partitioning stage in BCA. I tried erasing and just creating one partition, but I get this message: "An error occurred while erasing the disk."

Checking Disk Utility, it seems like it created the BOOTCAMP partition just fine.

But if I click Continue and try to copy over the Windows files, I'm back to this error: An error occurred while copying the Windows Installation files.

I tried the SMC and NVRAM reset, and I re-ran BCA.

No help. I'm at wit's end.

I don't know what else to try, except to return the computer for a refund, and go buy a Windows machine. Probably what I should have done in the first place.

The 1TB/6TB is nonstandard, yes. I bought this iMac from a reseller who added the 6TB drive. The 1TB is the OEM Apple SSD drive that comes as an upgrade with the new iMacs.

"Did any of the OS X installations come from a non-2015 Mac?"

No, I just downloaded El Capitan from the Apple Apps store and installed it yesterday. I'm using the Bootcamp version that came with the fresh install of El Capitan. Version 6.01 (522).

"Is it a TB-to-SATA/eSATA bridge for either of these drives?"

I'm not sure -- how can I check that?

Here's a few screenshots of the system reports for these devices:

I just talked to the reseller and asked him your question whether the TB-to-SATA/eSATA bridge was used for either device.

He says:

No. These models of iMac use a PCIe interface for the Flash (solid state) drive and SATA for the HDD. Not like the older models.

I assume you can see the System Reports I posted above for the two devices.

"Please also see https://help.apple.com/bootcamp/assistant/6.0/ ."

Yes, I read through that several times. BCA doesn't give me the option of not using a USB. When I first start up BCA, it asks me to pick a drive, which I do. Then it tells me: "The installer disc could not be found. Insert your Windows installer disc and wait a few seconds for the disc to be recognized." Of course, there's no optical drive on the machine. I have the ISO sitting on my Desktop, but it seems not to see that. So them I click "Go Back" and that's when I see the option to create a USB. So I figured that's what I was supposed to do.

Here's a screenshot:

OK, so I'm going to have to open up the machine and physically disconnect the 6TB hard drive? No way around that?

Sigh... I had the feeling that would be necessary.

I see quite a few videos about opening up these machines -- can you recommend a particularly good/reliable one?