Here follows a little addition.
Also after intensive research I was not able to install CentOS 7. However, I was finally able to install Kubuntu 18.04 LTS and it runs awesome nice on that old, old Apple hardware!
(Well, this may be caused most likely because of the newly installed 1TB Mushkin SSD, - whatever.)
What is my conclusion so far? The CentOS installer is unfortunately very
limited. For example, it does not allow a manual setting of the boot loader partition which is absolutely necessary in this context here. Furthermore the CentOS installer also rejects any non-standard conform MBR/GPT installation layout and blocks such installations. Also this is needed in this case with a hybrid MBR/GPT layout.
The Kubuntu/KDE installer allows the manual selection of the boot loader install partition. The installer is showing a warning when this is done. It notes that the installation will be most likely not bootable. There is furthermore also a warning that the "BIOS boot partition" is missing. Nevertheless the Kubuntu/KDE installer allows to continue the installation.
After the install is finished, it is necessary to boot up the first time through the rEFInd boot-loader (from CD, select EFI boot). There will be displayed Mac OS X and Linux. After selecting Linux, the iMac EFI settings will be automatically updated/corrected by rEFInd and Kubuntu is booting up. At the next restart, all bootable installations are available also from the stock Apple EFI boot-loader. Note, because Apple doesn't knows anything about Linux, - the Linux HD is named "Windows".
Important note, - you have to create and modify any disk partition layout changes always with the Apple disk utility. Do not use GParted for that because this will alter the MBR/GPT scheme. The Linux partitions must have the msftdata (Microsoft Basic Data partition) flag, this is necessary by Apple Boot Camp. The EXT4 root partition can have furthermore also additionally the "legacy_boot" flag. (This can be set later with GParted). According to the Microsoft perspective this makes the partition "active" and (MBR) bootable. Do NOT set the normal "boot" flag, only the EFI partition should have that flag.
My exact steps were:
1. Boot into Mac OS or a Mac OS install DVD, - run the Apple disk utility and set the preferred layout. On my 1TB SSD I decided me for:
Macintosh HD => 100GB (HFS+)
Linux HD => 892GB (exFAT)
swap => 8GB (exFAT)
Note, Mac OS 10.7 and higher will automatically add an OS X recovery partition, and also the EFI partition will be added if not present.
2. Boot into the Kubuntu Linux DVD. Select install, proceed until the partition configuration, and select manual.
Note, on older Apple computers the install DVD has to be remasterd to be bootable:
https://bugs.launchpad.net/ubuntu-cdima ... omments/16
3. Select the "Linux HD", chose modify and format it with EXT4, it will became then automatically root /.
4. Select the swap partition, chose modify and select swap file format.
5. Select the partition for the boot files manually to the "Linux HD" which is in my case Partition 4. Continue the install, ignore any warning messages.
After booting the first time with rEFInd, - Kubuntu should work great. Thanks to the Kubuntu proprietary driver install option, also the Broadcom BCM94360CS (BCM4360) 802.11ac + BT 4.0 combo card is working perfectly. And thanks to the "isight-firmware-tools" package, also the Apple webcam can be used in conjunction with the iSight firmware file. If the iSight doesn't work at the first time, - it may be necessary to boot into Mac OS and start the "Photo Booth" app. It seems that in certain situations the iSight firmware has to be loaded in Mac OS (which runs in native EFI mode). I noticed this issue also on older iMac computers under Windows. After that, the iSight webcam worked on Windows, and it is also working in Linux.
Some final words regarding the partition scheme. Interestingly, Linux seems to be capable to use also in legacy MBR mode more than 4 primary partitions. Windows is definitely unable to do this. However, thanks to this behavior, it should be possible to set "home" to a separate partition. The GPT layout will then look like:
Code: Select all
sda 1 EFI System Partition
sda 2 HFS+ (Macintosh HD)
sda 3 HFS+ (recovery HD)
sda 4 / (includes root, boot)
sda 5 home
sda 6 swap
As mentioned, this hybrid MBR/GPT layout will be limited at the MBR side to:
Code: Select all
sda 1 EFI System Partition
sda 2 HFS+ (Macintosh HD)
sda 3 HFS+ (recovery HD)
sda 4 / (includes root, boot)