How to make and use an USB stick with GParted that will boot on a Mac?
I will answer my own question. Since I had a lot of trouble creating Live USBs on Ubuntu and Mac and wanted to dig a bit deeper into the subject and will also list the output of the successful partition schemes.


Download .iso from here under "stable releases". If unsure, chose the 32bit-Version.
Put a USB stick in the slot and look into Disk Utility for its device number (disk1, disk2, diskX, etc). This can be found by clicking the icon "Info".
Open up a shell
Unmount the stick in terminal (nowhere else) with sudo diskutil unmountDisk /dev/diskX


Make sure you don't dd to a wrong disk, e.g. your working HDD!


Copy the downloaded file with sudo dd bs=1m if=file.iso of=/dev/rdiskX 


We use the r for raw in of=/dev/rdiskX to make the copy faster. You should not be using the s1 extensions: these are partitions on the disk, not the disk itself. 

The USB stick then looks like this with diskutil list:

/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *7.8 GB     disk1
   1:                       0xEF                         3.0 MB     disk1s2




sudo gdisk -l /dev/diskX will show this (aborting the question is possible with ctrlC):

GPT fdisk (gdisk) version 1.0.1

Partition table scan:
  MBR: MBR only
  BSD: not present
  APM: not present
  GPT: present

Found valid MBR and GPT. Which do you want to use?
 1 - MBR
 2 - GPT
 3 - Create blank GPT
Your answer:




Finally, sudo fdisk /dev/diskX will show 

Disk: /dev/disk1    geometry: 942/255/63 [15138816 sectors]
Signature: 0xAA55
         Starting       Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
------------------------------------------------------------------------
 1: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
 2: EF 1023 254  63 - 1023 254  63 [    535612 -       5948] &lt;Unknown ID&gt;
 3: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
 4: 00    0   0   0 -    0   0   0 [         0 -          0] unused 




OS X will show a dialog to Initialise..., Ignore or Unmount the USB stick when it is put into the slot. Clicking Ignore is safe. I put the German screenshot in, I think it's pretty clear.



When booting the Mac now press alt and there should be an option to boot to that is called EFI boot. This will bring up GParted Live.

But beware, there are some Macs (e.g. Macbook Pro 17" Early 2011 that are not able at all to boot anything other from USB than OS X).