I have downloaded Windows 10 on my Mac but I can't go back to Mac OS. When I hold down the option key it only gives me an option for Windows. I've tried making Mac OS active from Windows but it didn't let me. What should I do to get back to Windows.
iMac
You will need to
1. Follow How to install OS X on an external drive connected to your Mac - Apple Support and install a copy of OS X on an external disk.
2. You will need to instal GPT Fdisk - https://sourceforge.net/projects/gptfdisk/ .
Your OS X and Recovery HD partitions were modified to look like NTFS by Windows LDM. OS X will not boot from such a disk. Did you make any changes using Windows Disk Management to the disk.
This can also be done using Gdisk installed on the Windows side, if Windows is bootable, but it much easier using an external boot disk.
Unable to go to Mac from Windows 10
