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David Anderson
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This answer actually pertains to a this question and the closed question by mandesi c. Kaumba, which is titled Booting Linux on an 2006 iMac. The body of this question is given below.

I have removed a hard drive from my 2006 iMac and installed Ubuntu on it from another computer. However, after putting it back in the iMac, it is not able to boot. I am getting a question mark instead. Can anyone provide assistance?

The current 20.04 LTS release of Ubuntu has the following in the list of system requirements:

  • A 64 bit processor
  • At least a 2 GHz processor speed
  • 4 GiB of RAM (system memory)

None of the 2006 iMac models meet all of these requirements. While some models can accept 4 GiB of RAM, not all of this RAM can be accessed. The following excerpt was taken from everymac.com.

The net result is that at least 3 GB of RAM should be fully accessible, while when 4 GB of RAM installed, ~700 MB of of the RAM is overlapping critical system functions, making it non-addressable by the system.
 
Ultimately, 4 GB of RAM may be installed, but not all RAM in excess of 3 GB can be used due to the possibility of memory "overlap".

If your 2006 iMac has a 64 bit processor, then you might be able to install the current release of Xubuntu, which currently requires less RAM. However, all 2006 iMacs have a 32 bit EFI, so 64 bit operating systems can not be EFI booted from these Macs. This is the most likely cause of the problem described in mandesi c. Kaumba's question. However, for 2006 iMacs with 64 bit processors, there exists the possibility to BIOS boot 64 bit operating systems. However, below are some issues which need to be noted.

  • Many Linux installation DVDs (burned from Linux installation IS0s) contain more than one way to boot. For example, the DVD can be either EFI or BIOS boot. Many (if not all) 2006 iMacs can not boot from these types of DVDs.
  • Many (if not all) 2006 iMacs needed a firmware upgrade before being able to BIOS boot. To preform this upgrade, OS X needs to be installed on the Mac. Information on getting a Snow Leopard ISO file and creating an installer can be found here.
  • Some versions of Linux require the USB installer to be BIOS booted in order to install a BIOS booting operating system. The firmware for all 2006 iMacs can not BIOS boot from USB devices. However, the 32 bit Debian Linux USB installer can be 32 bit EFI booted and can install a BIOS booting Debian Linux. You can then use the Grub included with this installation to BIOS boot a 64 bit Linux installer (such as Xubuntu) and overwrite the 32 bit Debian with a 64 bit BIOS booting Linux. The procedure would be similar to what is post in this answer.

This answer actually pertains to a this question and the closed question by mandesi c. Kaumba, which is titled Booting Linux on an 2006 iMac. The body of this question is given below.

I have removed a hard drive from my 2006 iMac and installed Ubuntu on it from another computer. However, after putting it back in the iMac, it is not able to boot. I am getting a question mark instead. Can anyone provide assistance?

The current 20.04 LTS release of Ubuntu has the following in the list of system requirements:

  • A 64 bit processor
  • At least a 2 GHz processor speed
  • 4 GiB of RAM (system memory)

None of the 2006 iMac models meet all of these requirements. While some models can accept 4 GiB of RAM, not all of this RAM can be accessed. The following excerpt was taken from everymac.com.

The net result is that at least 3 GB of RAM should be fully accessible, while when 4 GB of RAM installed, ~700 MB of of the RAM is overlapping critical system functions, making it non-addressable by the system.
 
Ultimately, 4 GB of RAM may be installed, but not all RAM in excess of 3 GB can be used due to the possibility of memory "overlap".

If your 2006 iMac has a 64 bit processor, then you might be able to install the current release of Xubuntu, which currently requires less RAM. However, all 2006 iMacs have a 32 bit EFI, so 64 bit operating systems can not be EFI booted from these Macs. This is the most likely cause of the problem described in mandesi c. Kaumba's question. However, for 2006 iMacs with 64 bit processors, there exists the possibility to BIOS boot 64 bit operating systems. However, below are some issues which need to be noted.

  • Many (if not all) 2006 iMacs needed a firmware upgrade before being able to BIOS boot. To preform this upgrade, OS X needs to be installed on the Mac. Information on getting a Snow Leopard ISO file and creating an installer can be found here.
  • Some versions of Linux require the USB installer to be BIOS booted in order to install a BIOS booting operating system. The firmware for all 2006 iMacs can not BIOS boot from USB devices. However, the 32 bit Debian Linux USB installer can be 32 bit EFI booted and can install a BIOS booting Debian Linux. You can then use the Grub included with this installation to BIOS boot a 64 bit Linux installer (such as Xubuntu) and overwrite the 32 bit Debian with a 64 bit BIOS booting Linux. The procedure would be similar to what is post in this answer.

This answer actually pertains to a this question and the closed question by mandesi c. Kaumba, which is titled Booting Linux on an 2006 iMac. The body of this question is given below.

I have removed a hard drive from my 2006 iMac and installed Ubuntu on it from another computer. However, after putting it back in the iMac, it is not able to boot. I am getting a question mark instead. Can anyone provide assistance?

The current 20.04 LTS release of Ubuntu has the following in the list of system requirements:

  • A 64 bit processor
  • At least a 2 GHz processor speed
  • 4 GiB of RAM (system memory)

None of the 2006 iMac models meet all of these requirements. While some models can accept 4 GiB of RAM, not all of this RAM can be accessed. The following excerpt was taken from everymac.com.

The net result is that at least 3 GB of RAM should be fully accessible, while when 4 GB of RAM installed, ~700 MB of of the RAM is overlapping critical system functions, making it non-addressable by the system.
 
Ultimately, 4 GB of RAM may be installed, but not all RAM in excess of 3 GB can be used due to the possibility of memory "overlap".

If your 2006 iMac has a 64 bit processor, then you might be able to install the current release of Xubuntu, which currently requires less RAM. However, all 2006 iMacs have a 32 bit EFI, so 64 bit operating systems can not be EFI booted from these Macs. This is the most likely cause of the problem described in mandesi c. Kaumba's question. However, for 2006 iMacs with 64 bit processors, there exists the possibility to BIOS boot 64 bit operating systems. However, below are some issues which need to be noted.

  • Many Linux installation DVDs (burned from Linux installation IS0s) contain more than one way to boot. For example, the DVD can be either EFI or BIOS boot. Many (if not all) 2006 iMacs can not boot from these types of DVDs.
  • Many (if not all) 2006 iMacs needed a firmware upgrade before being able to BIOS boot. To preform this upgrade, OS X needs to be installed on the Mac. Information on getting a Snow Leopard ISO file and creating an installer can be found here.
  • Some versions of Linux require the USB installer to be BIOS booted in order to install a BIOS booting operating system. The firmware for all 2006 iMacs can not BIOS boot from USB devices. However, the 32 bit Debian Linux USB installer can be 32 bit EFI booted and can install a BIOS booting Debian Linux. You can then use the Grub included with this installation to BIOS boot a 64 bit Linux installer (such as Xubuntu) and overwrite the 32 bit Debian with a 64 bit BIOS booting Linux. The procedure would be similar to what is post in this answer.
Source Link
David Anderson
  • 35k
  • 11
  • 50
  • 93

This answer actually pertains to a this question and the closed question by mandesi c. Kaumba, which is titled Booting Linux on an 2006 iMac. The body of this question is given below.

I have removed a hard drive from my 2006 iMac and installed Ubuntu on it from another computer. However, after putting it back in the iMac, it is not able to boot. I am getting a question mark instead. Can anyone provide assistance?

The current 20.04 LTS release of Ubuntu has the following in the list of system requirements:

  • A 64 bit processor
  • At least a 2 GHz processor speed
  • 4 GiB of RAM (system memory)

None of the 2006 iMac models meet all of these requirements. While some models can accept 4 GiB of RAM, not all of this RAM can be accessed. The following excerpt was taken from everymac.com.

The net result is that at least 3 GB of RAM should be fully accessible, while when 4 GB of RAM installed, ~700 MB of of the RAM is overlapping critical system functions, making it non-addressable by the system.
 
Ultimately, 4 GB of RAM may be installed, but not all RAM in excess of 3 GB can be used due to the possibility of memory "overlap".

If your 2006 iMac has a 64 bit processor, then you might be able to install the current release of Xubuntu, which currently requires less RAM. However, all 2006 iMacs have a 32 bit EFI, so 64 bit operating systems can not be EFI booted from these Macs. This is the most likely cause of the problem described in mandesi c. Kaumba's question. However, for 2006 iMacs with 64 bit processors, there exists the possibility to BIOS boot 64 bit operating systems. However, below are some issues which need to be noted.

  • Many (if not all) 2006 iMacs needed a firmware upgrade before being able to BIOS boot. To preform this upgrade, OS X needs to be installed on the Mac. Information on getting a Snow Leopard ISO file and creating an installer can be found here.
  • Some versions of Linux require the USB installer to be BIOS booted in order to install a BIOS booting operating system. The firmware for all 2006 iMacs can not BIOS boot from USB devices. However, the 32 bit Debian Linux USB installer can be 32 bit EFI booted and can install a BIOS booting Debian Linux. You can then use the Grub included with this installation to BIOS boot a 64 bit Linux installer (such as Xubuntu) and overwrite the 32 bit Debian with a 64 bit BIOS booting Linux. The procedure would be similar to what is post in this answer.