Re: Read Error HDD

Yes, you'll still be able to boot into Windows 10 from your external SSD even if the internal HDD fails, as long as 

The SSD is properly configured as a bootable drive . 

 Your IMac's firmware/boot manager can still recognize and select it 

The failing internal HDD won't stop the system from running off the external drive, though you may want to disable it in windows to avoid hangs from OS trying to read it.