Hello Steve,
After you've downloaded the ZIP file, go into Utilities and open the Disk Utility.app.
1. Select the USB device
2. Select Partition (To create a partition on the USB drive)
3. Provide a name for this drive (GPARTED, etc.)
4. Select "MS-DOS" as the format. Make sure it's at least 300 MB
5. After this is accomplished, Its a good idea to go into Terminal and on the Command line enter: "Diskutil list"
7. Observe the partition tables created on the USB. (You should see at least 3)
a. Top USB drive will be /dev/diskX(2,3,etc.). (This is dependent on how many drives you have)
b. Under this you should see something like this:
/dev/diskX (2,3, etc.) (external, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *USB Size diskX
1: EFI EFI Partition Size diskXs1
2: MS-DOS FAT ?? USB Partition size diskXs2
0 = The boot sector
1 = The EFI bootable file
2 = Your MS-DOS partition
NOW: You can UNZIP the downloaded file on to the USB drive.
NOTE: A GPARTED script file should create the files necessary for the USB to boot. Since its configured as a MS-DOS drive.
A. Reboot your iMac while pressing the "Option" Key
B. You will be presented with different drives to boot from (Macintosh HD, Recovery, USB Drive, etc.)
C. Select the USB drive (GPARTED or whatever you named it) NOTE: The script file may rename it for you.
D. Your iMac should now boot from the USB and create a DOS environment with the user interface.
From here, follow instructions on the screen.
You should not need to use GRUB. Everything should be created on your Flash Drive. If you need to connect to the "Cloud", I think the program would do this for you. If not, you may get an error saying you're not connected to the Internet.
Most of the Instructions on GPARTED assume you're using a Windows PC. The GNU/LINUX is another OS which a lot of people don't use. GPARTED has written their application(s) to reach the largest user pool. A lot of people are not familiar with the MacOS Platform and the Utilities built into the OS. Using the Disk Utility to create the partition will eliminate the need to wade through these LINUX command line entries l
Here's the command line entries if you were creating a USB in LINUX:
fdisk -l /dev/sdd
Disk /dev/sdd: 12.8 GB, 12884901888 bytes
15 heads, 63 sectors/track, 26630 cylinders
Units = cylinders of 945 * 512 = 483840 bytes
Disk identifier: 0x000c2aa7
Device Boot Start End Blocks Id System
/dev/sdd1 * 1 26630 12582643+ b W95 FAT32
Then format the partition as FAT with a command such as "mkfs.vfat -F 32 /dev/sdd1"
WARNING! Executing the mkfs.vfat command on the wrong partition or device could cause your GNU/Linux not to boot. Be sure to confirm the command before you run it.
# mkfs.vfat -F 32 /dev/sdd1
mkfs.vfat 2.11 (12 Mar 2005)
GEESH.... It's a lot easier to use the Disk Utility on the iMac.
Best Regards
Jake