I have a 2017 iMac with an internal SSD. I have Bootcamp Windows 10 on an external USB 3 SSD. On OS X Sierra, I would Option-Startup and select the EFI drive and boot into Windows 10. I just upgraded the iMac to High Sierra and Option-Startup no longer works. No boot drive selection is displayed.
Try this, it may fix your problem. I have a SSD installed in a PCI slot on my old mac pro, I can boot to either OSX High Sierra or Windows but only by holding the option/alt key at bootup. Apple doesent currently support using bootcamp control panel to boot between them, due to High Sierra using the AFPS file system.
Thanks Loner T. I tried that and the very first re-boot holding the Option key gives the boot drive screen but only shows the Macintosh HD. Subsequent re-boots ignore the Option key.
I contacted Apple support and they are looking into it and are supposed to get back to me in a few days.
Yes, it shows BOOTCAMP Windows and the Macintosh HD macOS, 10.13.2 drives but when I select the Bootcamp drive and click Restart the message that the bless tool was unable to set the current boot disk comes up.
Support sent me an email stating no further help since an external boot drive is unsupported.
High Sierra has several security and EFI fixes, which break what you could do in Sierra. The external disk complicates this further. As a test, install Sierra on an external disk, and boot from it. Once booted in Sierra, select the external disk in Startup Disks and test.
I appreciate the test steps Loner T. I may give that a try and post the results. I have to admit that I am leaning towards abandoning the external Windows drive. It is just too much effort and Windows 10 would not allow a major update because it was on an external drive so ....
Do you know how safe putting Windows 10 on my iMac's internal, High Sierra SSD would be?
The current version of Bootcamp.exe does not understand the APFS structure. HS converts most solid-state disks to APFS, unless you use Using the macOS High Sierra OS installer’s startosinstall tool to avoid APFS conversion | Der Flounder .
I have the High Sierra version and having an issue to get windows to boot from an external SSD. After completing all the steps necessary to install windows on the SSD, when I tried to restart using the startup disk, the error came up "You can’t change the startup disk to the selected disk. The bless tool was unable to set the current boot disk." I tried the NVRAM reset, but it didn't do anything. Please help!!
External disk Windows installations are problematic. HS updated EFI on Macs.
Try a SMC/NVRAM reset and check with Alt/Option. Do you see the external disk in System Preferences -> Startup Disks.
I was incorrect about the subsequent re-boots ignoring the boot drive screen.
After you boot into macOS, does System Startup -> Startup Disks show the external USB3 disk as an option?
High Sierra has some specific issues when partitioning the internal disk, but all issues can be addressed. It is relatively safe. Regular backups are critical for a setup using the internal disk.
Disregard that link, apparently that is only for imac pro's.
I gave up on an external Win10 boot disk under High Sierra. I bit the bullet and let Boot Camp partition my internal SSD and installed it there. So far works very well and I do regular backups.
Option Startup not working since upgrading to High Sierra
