PureOS ISO on USB Unavailable for Boot on Mac OS X – Troubleshooting Guide

Note: This is from a comment added by the OP (who does have a working DVD drive): I've since successfully replaced OS X with PureOS from the DVD iso, installed with LVM full disk encryption, and grabbed the linux package firmware-brcm80211, which has enabled WiFi :)

You are correct. The iso file only contains the software to BIOS boot. I created a virtual machine using VirtualBox and tried both BIOS and EFI boot methods. Using the BIOS boot method, I was able to successfully install and boot using either a Master Boot Record (MBR) table scheme or a GUID Partition Table (GPT) scheme. When I tried the EFI boot method, the firmware could not detect any EFI files to boot from. After BIOS booting a PureOS install in VirtualBox, I was able to examine the ISO file and could not find any EFI files needed for EFI booting.

So where does this leave us. Well, Mac users know you can not BIOS boot from the USB. So the obvious solution would be to burn the iso to a DVD and boot from the DVD drive to install PureOS. If a DVD drive is not available, then there is another option involving Windows. The PureOS installation iso file contains software that will replace an existing Windows installation with a PureOS installation. Of course, there is the possibility that your model Mac has hardware that is not compatible with the version of PureOS you are trying to install.

A free trial version of Windows 10 can be downloaded from the Microsoft website Download Windows 10 Disc Image (ISO File). The website Installing Windows 10 without DVD or Third Party Tools gives a procedure for installing a BIOS booting Windows 10 from a USB flash drive. If this procedure does not work, then the older procedure given at this website Installing Windows 7 or 10 Pro 64 bit without DVD or Flash Drive should work. Once Windows 10 is installed, the procedure given below can be used to replace Windows 10 with PureOS.

I assume macOS is installed to a "Mac OS Extended (Journaled)" formatted partition. If your Mac has macOS installed using Core Storage or AFPS, then make the appropriate substitutions. I assume Windows 10 is installed for a BIOS boot on a single NTFS formatted partition. The goal here is to replace Windows 10 with PureOS. The macOS installation should be unaffected.

Note: For a better view of a image, either click on the image or open the image in a new window.

1. Boot to Windows 10.
2. Mount the pureos-8.0-live-amd64.hybrid.iso file.
3. Insert a 2 GB or larger flash drive in an USB port. Everything on this flash drive will be erased.
4. Open an "Administrator: Command Prompt" window.

5. Enter the following commands

   diskpart
   rescan
   list  disk
   

6. From the list of disks, determine disk number for the flash drive. Here, I will assume the number is 1. If your number is different, then make the appropriate substitution in the next command. Enter the following commands to create a single FAT32 formatted partition on the flash drive.

   select  disk  1
   clean
   convert  mbr
   create  partition  primary
   format  fs=fat32  label=pureos  quick
   

7. Use the output from the next command to determine the drive letters for the "PUREOS" and "PureOS 8.0" volumes. Here, I will assume the "PUREOS" volume has been assigned the drive letter E and "PureOS 8.0" volume has been assigned the drive letter F.

   list  volume
   

8. Enter the following command to quit the diskpart command.

   exit
   

9. Enter the following command to copy all the files from the "PureOS 8.0" volume to the "PUREOS" volume. If your assigned drive letters are different, then make the appropriate substitutions. This command will take a while to complete. Be patient.

   xcopy  /e  /h  f:\*  e:
   

10. Enter the following command to run the Debian-Installer loader.

    e:setup
    

11. When prompted, I selected the Normal mode. The images below were taken after choosing this path. If you have issues installing, you could try the Expert mode.

12. When prompted, select "Begin install on this computer.".

13. When the Debian-Installer loader has completed, select "Close". When prompted, select "Yes" to reboot.

14. When the image below appears, select "Debian GNU/Linux - Continue with install process". The computer should restart.

    

15. Proceed with the installation until an image similar to what is shown below appears. Select "Manual", then "Continue"

    

16. The next image should be similar to what is shown below. Remove the Windows partition(s). In this case, this would only be partition #4 on sda. So, highlight this partition and select "Continue".

    

17. In the image shown below, highlight "Delete the partition", then select "Continue".

    

18. In the image shown below, highlight "Guided partitioning", then select "Continue".

    

19. In the image shown below, highlight "Guided - use the largest continuous free space", then select "Continue".

    

20. In the image shown below, I highlighted "All files in one partition (recommended for new users)". You may desired a different choice. When done choosing, select "Continue".

    

21. In the image shown below, highlight "Finish partitioning and write changes to disk", then select "Continue".

    

22. In the image shown below, select the "Yes" button, then select "Continue".

    

23. Continue with the installation until you reach an image similar to what is shown below. Select the "Yes" button, the select "Continue".

    

24. In the image shown below, highlight the line containing "/dev/sda", then select "Continue".

    

25. Continue until installation is complete.

Final note: If you have problems booting PureOS, you may consider installing rEFInd to the EFI partition.