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my Windows drive started playing up. I was unable to select my Windows disk in the 'Startup Disk' preference pane. Mac OS just keeps telling me that "The bless tool was unable to set the current boot disk".

I had this problem just before Christmas, and did a bit of research on the internet. Long story short, I reset the pram and bingo! All cured now :)
 
Slight update:
Managed to get it start to boot (Winclone restored Win 10 image) from the SSD…then it froze. This was with EFI boot enabled by Winclone, no legacy option is working still.
The Windows 10 installer would boot from USB, but then goes into 'I can't install to the Windows partition that I, the Windows installer, have just formatted for you' mode…genius, Redmond, genius…and it can't do an upgrade install either because you have to boot from the volume to be upgraded.
Pretty sure now I'll have to settle for it working via VMWare Fusion, until the MP 7.1 comes out…
 
The Windows 10 installer would boot from USB, but then goes into 'I can't install to the Windows partition that I, the Windows installer, have just formatted for you' mode…genius, Redmond, genius…

That "can't install to the partition, blah, blah, blah" error is usually related to having external drives attached. I have a G-Raid drive connected via Firewire and if I forget to unplug it I get the same error. Once unplugged the error goes away and win can install to the partition you just created.

When installing windows it's always best to unplug everything that's not necessary, internal and external. It saves headaches....
 
That "can't install to the partition, blah, blah, blah" error is usually related to having external drives attached. I have a G-Raid drive connected via Firewire and if I forget to unplug it I get the same error. Once unplugged the error goes away and win can install to the partition you just created.

When installing windows it's always best to unplug everything that's not necessary, internal and external. It saves headaches....
Indeed. One of the (many) daft things is that the machine's configuration hasn't changed since it worked..at the end of December, unless having an SSD instead of HD as (attempted) Boot Camp drive counts. Same RAM, same PCIe cards, same HDs. All I did was buy a new SSD, swap that for the existing one on the PCIe card, set the new one up as High Sierra boot drive, then put the existing SSD in a sled for Windows. And hence this month of broken Windows. Barking mad.
 
Indeed. One of the (many) daft things is that the machine's configuration hasn't changed since it worked..at the end of December, unless having an SSD instead of HD as (attempted) Boot Camp drive counts. Same RAM, same PCIe cards, same HDs. All I did was buy a new SSD, swap that for the existing one on the PCIe card, set the new one up as High Sierra boot drive, then put the existing SSD in a sled for Windows. And hence this month of broken Windows. Barking mad.
Ah, the old "I didn't change anything" immediately followed by "All I did was..." problem. :)

My suggestion is to go back to the very basics and try to get a working installation of Windows. Remove all PCIe cards except for the video card (if available install the original Mac card). Remove all SSDs and put a spinner back into the first slot. Then attempt to install Windows using a DVD (more reliable than USB as I've had problems with some USB drives failing as installation media).

Since I didn't read every post hopefully this isn't redundant.

EDIT:

I see you've tried some of this. You may want to try the following:

Remove every hard disk except the one you want to install Windows to. This will make the following easier and potentially avoid loss of data from working with the wrong drive.

1. Boot the Windows installation media using the EFI option
2. Choose your keyboard, language, etc. and click "Next"
3. Select "Repair Your Computer"
4. Select "Troubleshoot"
5. Select "Command Prompt"

The following are all typed within the command prompt window (without the quotation marks):

6. Type "diskpart"
7. Type "list disk"
8. Type "select disk n", where n is the disk you wish to install Windows to
9. Type "clean"
10. Type "exit"

Reboot the system and try installing Windows using the EFI option.
 
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Ah, the old "I didn't change anything" immediately followed by "All I did was..." problem. :)

My suggestion is to go back to the very basics and try to get a working installation of Windows. Remove all PCIe cards except for the video card (if available install the original Mac card). Remove all SSDs and put a spinner back into the first slot. Then attempt to install Windows using a DVD (more reliable than USB as I've had problems with some USB drives failing as installation media).

Since I didn't read every post hopefully this isn't redundant.

EDIT:

I see you've tried some of this. You may want to try the following:

Remove every hard disk except the one you want to install Windows to. This will make the following easier and potentially avoid loss of data from working with the wrong drive.

1. Boot the Windows installation media using the EFI option
2. Choose your keyboard, language, etc. and click "Next"
3. Select "Repair Your Computer"
4. Select "Troubleshoot"
5. Select "Command Prompt"

The following are all typed within the command prompt window (without the quotation marks):

6. Type "diskpart"
7. Type "list disk"
8. Type "select disk n", where n is the disk you wish to install Windows to
9. Type "clean"
10. Type "exit"

Reboot the system and try installing Windows using the EFI option.
Helpful stuff. Still didn't work, sadly. I left all the cards in, and before anyone starts telling me that's wrong, they were all in there in the first place (in the case of the USB 3.0 card & SSD card) and when it was last working (GT 120 & RX 580). Not just running, clean installs as well. So there, etc. Found (well, probably re-found) the correct way to get Windows to format the drive- delete all the existing partitions and then it actually works- but the installer then gives an error message about being unable to prepare the computer to boot into the new installation...This is off both DVD and USB installers, both SSD and HD did the same thing. Meh.
 
Helpful stuff. Still didn't work, sadly. I left all the cards in, and before anyone starts telling me that's wrong, they were all in there in the first place (in the case of the USB 3.0 card & SSD card) and when it was last working (GT 120 & RX 580). Not just running, clean installs as well. So there, etc. Found (well, probably re-found) the correct way to get Windows to format the drive- delete all the existing partitions and then it actually works- but the installer then gives an error message about being unable to prepare the computer to boot into the new installation...This is off both DVD and USB installers, both SSD and HD did the same thing. Meh.
The recommendation to remove the cards is to reduce the number of variables possible. Leaving the cards in just adds to the potential sources which could be causing your issue.

Something changed...it used to work, now it doesn't. Since there's no harm in removing them why not do so? Start with the most basic configuration you can to see if you can get it to work. If it does then you know something in the additional configuration is causing a problem. If not then something in the base configuration is the problem.
 
The recommendation to remove the cards is to reduce the number of variables possible. Leaving the cards in just adds to the potential sources which could be causing your issue.

Something changed...it used to work, now it doesn't. Since there's no harm in removing them why not do so? Start with the most basic configuration you can to see if you can get it to work. If it does then you know something in the additional configuration is causing a problem. If not then something in the base configuration is the problem.
As it worked with all those cards in place, including when and after I did a clean install of Windows, I am not inclined to take them out on the off-chance (and, frankly, that seems highly unlikely after everything I've tried) that might work- not least because it might screw up again when I put them back. Nope, I think I'm done. Not being able to boot into Windows is a minor irritant, but I can do without. Certainly less bother than trying to fix it has been. I'll stick with using Fusion if I need Windows for the time being…
 
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