Q: bootcamp fails to start after upgrade to macOS Sierra

Yes, I can.

Is that good? Bad?

BTW, this is a mid-2011 iMac with one Sierra partition and one Windows 8.1 partition that was working perfectly on Mavericks.

Can you use Re: bootcamp fails to start after upgrade to macOS Sierra to update your MBR? Was your original iMac a Fusion drive, that you split manually?

Nothing happened...

I mean. I reboot holding option, choose the Windows partition and nothing happens. I then have to hold the power button to turn the iMac off and on again to boot Mac OS.

disk 0 is a regular 1TB hd that came with the iMac. disk1 is an ssd I installed in place of the superdrive.

Can you post the output of

sudo fdisk /dev/disk1

Disk: /dev/disk1 geometry: 14593/255/63 [234441648 sectors]

Signature: 0xAA55

         Starting       Ending

#: id  cyl  hd sec -  cyl  hd sec [     start -       size]

------------------------------------------------------------------------

1: EE 1023 254  63 - 1023 254  63 [         1 -     409639] <Unknown ID>

2: AF 1023 254  63 - 1023 254  63 [    409640 -  170205096] HFS+       

3: AB 1023 254  63 - 1023 254  63 [ 170614736 -    1269536] Darwin Boot

*4: 07 1023 254  63 - 1023 254  63 [ 171884544 -   62556160] HPFS/QNX/AUX

This may not fix the issue, but please run

Reset the System Management Controller (SMC) on your Mac - Apple Support

How to Reset NVRAM on your Mac - Apple Support

If this does not work, can you swap the SSD and HDD so that Bootcamp is in the Main Bay?

I'll do what you asked, but swapping the hard drives is not possible. I'm not confident enough about opening the iMac by myself.

Just resetted both the SMC and NVRAM. Still can't boot.

Under which version of OS X did this set up work correctly? Mavericks?

Yes. I'm afraid I'll have to downgrade and do all the process again.

I could only make this windows installation work by doing it in vmware fusion, then mounting the image and cloning via winclone. As soon as it's upgraded, it stops booting.

gugalpm wrote:

 

Yes. I'm afraid I'll have to downgrade and do all the process again.

You can install the older version of OS X on an external disk, without rolling back the OS X version on the internal disk. Please see How to install OS X on an external drive connected to your Mac - Apple Support .

Will that change how the bootcamp partition in my internal drive will work? Seems like something to try.

It will save your internal disks from being modified.