Thanks for the suggestion, @kyte Yes I tried that one out. Does a byte by byte copy apparently… but also produced a non-bootable USB. 
Tried Fat32, and ExFat…
Win10 installer loads on my sons laptop.
SSD connects via dock.
But, Win10 installer only wants to instal on the built in drive…
I swear someone is laughing at me…
You should not need to decide the format…
From the research I’ve just done… Windows doesn’t like to be installed on an external drive…??? Have just been looking at various ways to get around this, none simple. (Ok - maybe simple, but many involving downloading 3rd party software which on a PC always worries me…)
Am about ready to give up for now. My son has said if I can’t get windows running, he wants to go up to High Sierra, as that will unlock access to a few games he wants on Steam. I believe I ran High Sierra briefly on the MacPro and it was ok (unlike Mojave) so will do that… and next time I head to my father in law’s house, I’ll take the SSD with me and get him to sort it out for me.
Cosmic, HS should run fine on the 5 1 Mac Pro. I run HS on the late 2011 2.2GHz Quad core i7 16GB RAM 15 inch MBP I’m typing this on. - My old warhorse, several dents, missing bottom case screw, busted Discrete GPU - now permanently powered down, a 500G SSD + 1TB HD in the Optical drive bay.
The 5 1 Mac Pro is only a year older than this old bus. - Seriously give the free VMWare Fusion Player a go, or Parallels in its try before buy set up. You’ve already easily spent the cost of a standard Parallels licence (VM’s limited to 8G RAM in non subscription licences though) in your time getting nowhere so far… 
Did you try OCLP?
@jaysee
I had a look in their FAQ, but couldn’t see any info there RE installing Windows? I’m not having issues booting Windows on the MacPro - I can’t even instal Windows onto the MacPro, or a USB, or an SSD…
@JimWOz
Yeah I thought HS would be ok… and then he can play Barrowtrauma (?) and be happy, at least for the time being. And ok… not getting anywhere with installing Win10 so might try VM… Used to run it on my MacBookPro. Or was that Parallels…
I think a VM on the Mac Pro would be too slow to play games. I’d try @jaysee’s advice next & give OCLP a go. Also, like I said before, remove the optical drive & see if it’ll let you install windows from the disk image.
@jaysee - sorry ok, followed your original link RE OCLP, which has a link specific for MP5,1… there’s a LOT of instructions there… will make some time to read through and see if I can follow it. If I’m reading correctly, it’s saying to have both Mac OS and Windows on the 1 drive. Had planned on having 2 different drives - Win and Mac - but it’s not a “must”.
So… the 14yo wanting Windows is now 15yo… and becoming “isolated” cos a lot of the Steam games he wants to play with his friends are now not High Sierra compatible (and Mojave was unusable when I tried it).
This morning I decided to revisit Windows 10, and rather than trying to make the USB installer to make a Windows-only SSD, try the option that seems to be more “Apple compatible” - start with MacOS and make a windows partition. Most the issues above are related to getting the USB installer set up… so - maybe that will solve all.
I’d taken the 500GB SSD out of the MacPro and was trying to set it up with my Mac Mini via my USB HDD dock… so I got that all set up again, turned it on, and - it’s dead. LOL Bloody Hell!
Will put the SSD back into the MacPro, instal High Sierra on it, and then try create a (large) Windows partition, so HS is just taking up as minimal space as possible… Now just need to go wake the 15yo…
It can definitely be on a dedicated drive. That’s how I have it set up on two 5,1s I use — one with Windows 10 for Parsec remote gaming to play old/Windows games on M1 Mac over network, the other as a Windows 10 HyperV vHost test lab at work.
Yeah - that would be my preference, but I can’t for the life of me work out a way to get the USB install stick set up - fails every time.
Now High Sierra is failing to install, just to add to my joy.
High Sierra now installed on the SSD inside the MacPro… but I think my glorious plan to use Boot Camp has gone full circle. Can’t see comments on this above - maybe was in another thread… Bootcamp will only offer to install Windows 7 or 8 - not 10.
Bootcamp version is 6.1, and according to what I can see online, it should offer Win10. But it’s not. Have the same version on my Mac mini, and it offers Win10.
Will need to try work out if I can get my HDD dock working so I can use it to boot the Mini, and then try bootcamp again…
Good luck…
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This might seem like a really dumb idea but if you have a DVD burner in the Mac Pro, take it out & see if that makes any difference to Bootamp giving you access to install Windows 10. Just clutching at straws now…
Thanks - but - Crisis Averted!
He bought a Dell Optiplex 9020 from his school.
New Crisis - Need to install Win10 on an Optiplex 9020!
Have no idea on the specs, but gather it’s from around 2013 based on a quick web search. Lil bugger heard the school was selling them for $50 and scared hell out of his mum this afternoon calling saying “I have something to tell you!! I need you to come pick me up and bring $50!” LOL
Will probably start a new thread, if I can bring myself to defile AppleTalk with a PC thread… Cos I’m sure I’ll still need help. And sure he’s going to be doubtless spending more money on RAM, sound card, graphics card… Hopefully it’s got at least an i5 inside…
What are you going to do with your Mac Pro now?
No real justification for keeping it so probably ebay.
That’s a pity. I still regret selling mine. Even if it wasn’t getting any use…
Lol
Don’t let my wife hear you encouraging me to keep it.
I think I’ll need to invest in a better HDD tower at some point… the “Icybox” is doing ok, but I do get a disconnection about once a week. Dunno if it’s the box or the mini.