How to create a second partition on an external disk and is it safe?

Mac OS X's Time Machine backup utility won't work with FAT32.
It is recommended that you choose a Time Machine destination that’s roughly two-to-four times the size of the drive you are backing up.
FAT32 is an older file system that’s largely relegated to USB flash drives and other external drives.
Windows uses NTFS for its system drive, and it’s also ideal for other internal drives.
exFAT is a modern replacement for FAT32, and more devices support it than do NTFS — although it’s not as widespread as FAT32.

FAT32 file system.
This type of a file system has a built-in limitation on the size of the files that it may contain.
Although the total size of the files that you can copy to a FAT32 drive could be as large as 2TB (or the physical capacity of the drive, whichever is smaller), the size of each individual file may not exceed 4GB.

hard drive - How to create a second partition on an external disk and is it safe? - Ask Different
I want to create a second a partition in my hard disk where one (about 900 GB) is used for Time Machine and another (100 GB) is used to store info like a thumb drive.

I don't want any existing data to be lost. I know someone who partitioned his hard drive and it is now spoiled and not repairable. I have important info in this hard drive and this is my only one.

For your info, I am using a retina MacBook Pro with Mavericks and want to repartition a WD My Passport for Mac hard disk.

Yes you can repartition without losing data.
Using Disk Utility, perform a repair on your drive to make sure the drive is free of errors (even better, use Diskwarrior if you have a copy).
Then unmount your drive but don't eject it.
Select the drive in the left hand pane, then go to the Partition tab.
On the Partition Layout section click on the "+" to create a new partition.
Optionally you can specify the new partition size.
When you're happy hit the Apply button and wait.

If you've absolutely critical data on the drive make sure you have a backup (which you should have anyway) but I've done this many times without an issue.
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Mount your hard drive and launch Disk Utility.
In the drive pane select the hard drive you want to partition (not the volume on the drive but the drive itself).
Click on the Partition tab that appeared when you selected the drive.

The Partition Layout area will show you a graphic representation of your hard drive—it will be a single partition partly colored blue (the blue indicates the portion of the drive that contains data, but not its location on the drive).
Your first step is to resize this partition.
You can do that by dragging up on the bottom right corner of this image (where you see the three gray lines) or you can simply enter a new value in the Size field (700GB, for example).

Now click the Plus (+) button below the partition to add another partition.
By default it will consume the remaining space on the drive.
You can add more partitions, if you like, by clicking on the Plus button.
To change their size use the Size field.

When you’re ready to partition the drive, click on the Apply button. (If you change your mind, click on Revert to put things back the way they were.)
Disk Utility will go about its business, making sure that your data remains on the first partition as well as creating the additional partition.
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One of the first things to take into account is that the drive is likely preformatted to FAT32 instead of a more OS-specific format.
Since FAT32 is readable and writable on both Windows and OS X systems, this is quickest way for a drive to work on each platform; however, it does have some limitations, including the lack of journaling support that would help prevent data corruption, and lack of support for filesystem permissions.

if it is a single-drive device, then before using it be sure to format it using Disk Utility in OS X.
If the drive is going to be used only with your Mac and with other Macs, then consider using Apple's Mac OS X Extended (Journaled) format, and only use FAT32 if you intend to use the drive with a Windows system.
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You can share an external drive between Mac and windows by formating your drive either by using any of the following file format given below:
a)FAT or FAT32(win old format)
b)NTFS(windows new format)
c)HFS+ OS X Extended journaled(Mac format)
Depending on two issues:
- 1.file size limitation issue: Fat32 has 4GB max file size limitation while NTFS and HFS+ have no file size limitation.
2.Read/write issue: Fat32 is read and writable on both Win/Mac.
NTFS readable on both mac/win, but writable on Mac only.
So if you don't have bulky videos you can choose FAT32 format.
So, for large sized files you should opt for HFS+ format.
HFS+ have no file size limitation, but it also read/writeable on macs.
I want to suggest you something that there is no need to go for third party tool even if you want to format your drive using HFS .
You can make it read/write accessable by format it using HFS and use HFS Explorer to read/write on windows.
And if you want to format your external drive using NTFS, then use NTFS-3G to read/write on Mac.
Specifically in this case, you would have to format your external Hard Drive either in FAT or FAT32 file or in ex-FAT format, it will allow your data to read and write in both OS.
In multi-operating systems ex-FAT is best, but it doesn't mean this doesn't have any disadvantage.
Ex-FAT have no issue regarding file size but with speed it somehow slow while transferring files.

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You can share an external drive between Mac and windows by formating your drive either by using any of the following file format given below:-
a)FAT or FAT32(win old format)
b)NTFS(windows new format)
c)HFS+ OS X Extended journaled(Mac format)

Depending on two issues:-

1.file size limitation issue:
Fat32 has 4GB max file size limitation while NTFS and HFS+ have no file size limitation.

2.Read/write issue:

Fat32 is read and writable on both Win/Mac.
NTFS readable on both mac/win, but writable on Mac only.

So if you don't have bulky videos you can choose FAT32 format. So, for large sized files you should opt for HFS+ format.
HFS+ have no file size limitation, but it also read/writeable on macs.
I want to suggest you something that there is no need to go for third party tool even if you want to format your drive using HFS+. You can make it read/write accessable by format it using HFS+ and use HFS Explorer to read/write on windows.
And if you want to format your external drive using NTFS , then use NTFS-3G to read/write on Mac.
Specifically in this case, you would have to format your external Hard Drive either in FAT or FAT32 file or in ex-FAT format, it will allow your data to read and write in both OS. In multi-operating systems ex-FAT is best, but it doesn't mean this doesn't have any disadvantage. Ex-FAT have no issue regarding file size but with speed it somehow slow while transferring files.
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FAT32 (File Allocation Table)
- Natively read/write FAT32 on Windows and Mac OS X.
- Maximum file size: 4GB
- Maximum volume size: 2TB

NTFS (Windows NT File System)
- Natively read/write NTFS on Windows. - Read-only NTFS on Mac OS X
- Native NTFS support can be enabled in Snow Leopard and above but has proven instable.
- Maximum file size: 16 TB
- Maximum volume size: 256TB

HFS+ (Hierarchical File System, aka Mac OS Extended)
- Natively read/write HFS+ on Mac OS X
- Required for Time Machine
- Maximum file size: 8EiB
- Maximum volume size: 8EiB
The best ways to format an external drive for Windows and Mac - CNET
Option 3: Create two partitions on your hard drive to use with each OS, separately.
This solution is a little different than the previous two because instead of having one hard drive that works with both machines, you're splitting your HD into two sections, each dedicated to a different OS. For example, if you have a 1TB hard drive, 500GB of storage can be used with your Windows computer, and 500 will be dedicated to your Mac computer.

You won't be able to write to the Mac side from your Windows computer, and vice versa, but it's a good solution for people who want all the advantages each format has to offer for its respective system. Here's how to do it:
The best ways to format an external drive for Windows and Mac - CNET