precontitions:

    author has limited english knowledge
    27" imac i5, 11,1, late 2009
    1 TB harddisk
    ati mobility radeon hd 4850
    linux mint 11 dvd
    working internet connection
    refit 0.14 (macosx image .dmg) installed
    (at least 250GB) free space on the harddisk

installation:

1.     backup all your data
2.     open disk utility in snow leopard (Application - Utilities), go to partiton, go to volume scheme, go to the lower right corner of the volume scheme and move the lower border upwards
3.     configure the new space as free space and hit apply
4.     install refit 0.14 within macosx
5.     download linux mint 11 dvd 32bit and burn it to a dvd
6.     reboot your imac (i had to rebbot twice to get into the refit menu)
7.     put in your linux mint 11 dvd and reboot your imac again (the boot process is slower with refit)
8.     choose in the refit menu the linux mint live dvd
9.     IMPORTANT: hit the TAB-key in the boot option menu of the linux mint live dvd
10.     after hitting the TAB-key yout get a command line with two minus signs --
11.     use the backspace-key to remove the two minus signs and set the following two boot options: nomodeset xforcevesa
12.     hit the return-key and wait
13.     you should get into the linux mint desktop; but screen resolution will not be perfect; but you can install linux mint.
14.     If you have a dualboot system (like i have) do not care; linux mint will do a fine job. simly choose the option linux mint beside macosx.
15.     IMPORTANT: when the linux mint install procedre ends, do not reboot your imac!!!!
16.     go to the linux mint menu and start the application terminal (for the following install procedure you need a command line, sorry).
17.     all commands you have to set, i will write down in red color and bold
18.     command 0: sudo su
19.     command 1: cd /
20.     command 2: ls -l
21.     you should see a directory target
22.     go to the linux mint menu and look for GParted (partiton tool on the live dvd), start gparted then look for the linux partitons /dev/sdaX (X placeholder for a numeric character). there should be three new linux partition. write down /dev/sdaX of the biggest linux partition (not boot_grub, not linux-swap). Tipp: if you never used GParted run it before and after the linux mint installation, so you can see the befor and after. IMPORTANT: DO NOT CHANGE ANYTHING IN THE PARTITON SHEME!!!
23.     command 3: mkdir -p /target/sdaX (X placeholder for a numeric character)
24.     command 4: mount /dev/sdaX /target/sdaX
25.     command 5: cd /target/sdaX/boot/grub
26.     check if the grub configuratiuon file has an entry quiet
27.     command 6: less grub.cfg | grep quiet
28.     the output of the command 6 should show a single line with word quiet within; if this is the case, we are going to modify this line; if not i can't help you.
29.     command 7: sed -i 's/quiet/quiet nomodeset xforcevesa/g' grub.cfg
30.     check if the grub configuration file has an entry single
31.     command 8: less grub.cfg | grep single
32.     the output of the command 8 should show a single line with the word single; if this is the case we are going to modify this line; if not i can't help you.
33.     command 9: sed -i 's/single/single nomodeset xforcevesa/g' grub.cfg
34.     command 10: cd /target/sdaX/etc/default
35.     check if the file grub has an entry quiet
36.     command 11: less grub | grep quiet
37.     if yes, we are going to modify this line. otherwise ... sorry.
38.     command 12: sed -i 's/quiet/quiet nomodeset xforcevesa/g' grub
39.     check if the file grub has an entry GRUB_CMDLINE_LINUX
40.     command 13: less grub | grep GRUB_CMDLINE_LINUX
41.     if yes, we are going to modify this line. otherwise ... sorry.
42.     command 14 (on a single line): sed -i 's/GRUB_CMDLINE_LINUX=""/GRUB_CMDLINE_LINUX="nomodeset xforcevesa"/g' grub
43.     command 15: cd /
44.     command 16: umount /target/sdaX (don't forget X placeholder for a numeric character)
45.     shutdown your imac / eject the linux mint live dvd
46.     start your imac; choose the linux partition in the refit menu (wait the boot porcedure take a lot more time); you will get into the linux mint boot menu; hit enter or wait some seconds
47.     wait until yo can see the linux mint login window; log in
48.     unfortuantelly the screen resolution is stil wrong but there is no black screen ,-)
49.     update your linux installation (you should see the linux mint update icon ont the lower right corner of your desktop.
50.     there should also be an icon, that reminds you to install a proprietary driver. it's the amd/ati driver for your graphic card (radeon hd 4850). install it.
51.     if there isn't an icon for the amd/ati driver go to amd/ati driver hompage and download the latest graphic driver. install this driver: a) apt-get install build-essential b) bash ./ati-driver-installer-11-9-x86.x86_64.run
52.     go back to the termial and create and/or modify xorg.conf
53.     command 17: /usr/bin/aticonfig --initial
54.     command 18: cd /etc/X11
55.     command 19: ls -l
56.     there should be a file xorg.conf (generated by the command 17) and an older version xorg.conf.original-0
57.     backup the xorg.conf
58.     command 20: cp xorg.conf xorg.conf-atiorig
59.     go to the linux mint menu an run gedit; then open xorg.conf.original-0
60.     go back to the terminal and open the xorg.conf using nano (editor)
61.     command 21: nano xorg.conf
62.     remove all lines in xorg.conf you also find in xorg.conf.original-0. to do that set the coursor at the beginning of the line you have to delete; then hit control-key k-key.
63.     when you remove all the lines you have to delete, hit control-key x-key nad answer the following question (save) with yes.
64.     close Gedit, close Terminal, close GParted, shutdown your imac
65.     start your imac again, wait, choose the linux partition in refit, wait, hit enter in the linux mint boot menu, wait, login.
66.     NOW YOU SHOULD HAVE LINUX MINT IN THE WRIGHT SCREEN RESOLUTION.

THANKS TO: supidupi (ubuntuforum), jack sparrow (my blog), kulbir saini (fedora howtos, tutorials, tips and tricks)

kind regards, go4unkwn