Can't modify external hard drive partition with Disk Utility

Hi,
I have an external 750GB hard drive that I previously formatted as FAT 32 because I wanted to be able to plug it into Windows computers as well. I was not very thoughtful at the beginning and I just created one big 750GB partition. I currently have 450GB used and about 300GB free.
I would like to create a new partition in the free space without losing the data I have in the disk, and format the new partition as Mac OS Extended to use it for Time Machine backups. Presumably Disk Utility can do it, but when I go to the "Partition" tab it says "This partition cannot be modified" and the "+" sign below it is grayed. Can anybody help me repartitioning my drive without losing my data?
Thanks!!!

That would likely be because the drive is partitioned MBR and not GUID. You cannot repartition an MBR partitioned drive on the fly. What you need is a drive that is partitioned using GUID (GPT) with on partition formatted for the Mac and the other formatted FAT32. But to do that you must start the drive from scratch:
Drive Preparation
1. Open Disk Utility in your Utilities folder.
2. After DU loads select your hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Click on the Partition tab in the DU main window.
3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Set the format type to Mac OS Extended (Journaled.) Click on the Options button, set the partition scheme to GUID then click on the OK button. Click on the Partition button and wait until the process has completed.
4. Select the entire partition in the rectangle by clicking on it (the lines will turn darker blue) and click on the [+] Add button. The volume will be divided in two. Select each volume and resize as your require.
5. Select one of the volumes you just created. Set the format type to Mac OS Extended (Journaled.) Click on the Erase button.
6. Repeat Step 5 using MSDOS as the format type.
This will work with any PC that understands the GPT partition scheme which is Vista and Windows 7. Otherwise, you should just leave the drive as it is and forgo using Time Machine or purchase another drive for Time Machine.

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Ejectable: No
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Ejectable: No
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Analyzing File Structures
Passed
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Extents File (B-Tree) <Passed>
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Catalog File (B-Tree) <Passed>
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Passed
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Format: HFS+ (Mac OS Extended)
Journaling: Enabled
Case-sensitive: No
Created: 03/26/07 01:26:09 AM
Modified: 03/26/07 01:26:09 AM
Capacity: 152.54 GB
Used: 49.81 GB
Available: 102.74 GB
Files: 86,074
Folders: 16,342
Writable: Yes
Ejectable: No
Removable: No
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Passed
I'll let you know if Tech Tool Pro's surface scan of the drive turns anything up (it's running right now - takes several hours to run), but hopefulyl this provides a good starting point into painting a picture of my system...
Thx - Bob

Can't modify external hard drive partition with Disk Utility

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