Hello and welcome to my User Tip.
I will explain to you just the basics how storage drives operate in regards to different operating systems, formatting and partitions, especially sharing data with Windows PC's which is the most common asked about issue.
For simplicity sake, I have stuck to the most current used formats and steps on Mac's and PC's that your likely to encounter and encouraged to use now.
Whenever you changing the drive, there is the potential for the lost of data.
So make sure you have a copy of the data off the drive your attempting to change and physically disconnect all other non-essential drives so no mistakes occur.
#1 If you want to format a drive, USB Thumb drive etc., to only work with a recent Mac and to boot OS X with
Open Disk Utility and select the drive makers name and size on the far left, then Partition: Options: GUID partition table and Format: OS X Extended (Journaled) name it and click Apply.
#2 Format a drive, USB thumb drive etc., to work with Mac or a PC, on a PC. (most compatibility)
Take it to the oldest Windows PC your going to share data with, usually XP, right click on it's icon and select format:
exFAT - for large video/movie files use +4GB (pre-10.6.5 Mac's can't read this format, use FAT32)
FAT(32) - for most anything, small use (under 4GB sized files), thumb drives, most compatibility with connecting to TV's consoles and other devices
If your Windows XP machine doesn't have exFAT, there is a free download from Microsoft.
#3 Format a drive or USB thumb drive etc., to work with Mac or PC, on a Mac (most compatibility)
Take the drive to the oldest OS X version on the Mac(s), Open Disk Utility and select the drive makers name and size on the far left, then
Partition: Options: MBR partition table
Format: MSDOS (FAT32) - for under 4GB files and to work work with consoles, devices that need firmware etc., or
Format: exFAT - for over 4GB sized files (Mac's pre OS X 10.6.5 won't have exFAT)
#4 Format a drive or USB thumb drive etc., to work with only between recent Mac 10.6.5+ & Vista-Win 7, on a Mac
Open Disk Utility and select the drive makers name and size on the far left, then
Partition: Options: GUID partition table
Format: MSDOS (FAT32) for under 4GB sized files, or
Format: exFAT - for +4GB sized files (exFAT is proprietary, patent pending, may have to pay later)
Trouble with transfering data or formatting?
Partition Map or Table - is instructions written on the drive that tell the hardware it's running from: what, how many, and where the partitions are on the drive. There is only one partition map on the drive. It can be MBR or GUID.
Partition - is a space on the drive that appears as another drive, but still resides on the same piece of physical hardware.
Formatting - is how a entire drive or partition is structured to hold data, one drive can be partitioned into many separate partitions each with their own formatting. The Partition Map holds all the information of what is what.
Changing the partition map/table scheme will erase all data on the entire drive, it's like burning a treasure map, no directions anymore.
Changing the format of a single partition on a multi-partitioned drive only erases the data on the partition being formatted.
How do I format a entire external drive on a Windows PC?
Open the file manager and locate the drive (empty of data, or data you wish to delete) and right click on it and "format" and choose a format, Windows will create the MBR or GUID automatically if there is none depending upon what version of Windows your using.
It's best to format on the oldest PC being used as the future versions of Windows (and Mac's) can read the older format.
I have a drive from a Windows PC, the Mac can/not read or not write to it. What do I do?
It's likely formatted NTFS by Windows, it's best to take it back to the Windows machine, remove the data and reformat it FAT32 or exFAT, then place the data back on.
Recent Mac's can read NTFS, but not write to it, so this might be enough to transfer data off, then reformat the drive. Check the data was transfered and is accurate before changing the drive format and thus erasing it.
There is third party software than can be installed enable read/write of the NTFS format on Mac's, you'll need to search online for it. It's hassle, you'll have to pay and have it updated like any other software. A neutral exFAT or FAT32 (MSDOS) format is better as either platform can read it without more software installed.
How do I format a entire external drive on a Mac?
Unmount and disconnect all other drives to prevent a mistake. Open Disk Utility in the Applications/Utilities folder and select the drive on the far left with a makers name and size, then click Partition tab: Options: (for a establishing a specific partition map scheme), partitions, and Format (for selecting a specific format for the drive/partitions)
How do I format a drive for booting with a Mac?
Same steps as above, choose GUID as the Partition map under Options, and OS X Extended journaled (HFS+) as the format for the partition your going to be installing OS X into.
There is no choice for MSDOS format on the PC, or FAT32 on the Mac, what do I choose?
FAT32, if it just says FAT, it usually means FAT32 (as FAT16 is on the way out.). MSDOS is what Apple calls FAT32.
I formatted the external drive exFAT or FAT32 (MSDOS) on my Mac and took it to a PC and it asks me to format!
Don't format it (unless your willing to erase all the data on it)
Take the drive back to the Mac and copy off the data (if you don't have a copy on the Mac already) and try formatting it again, this time making sure to select the Option: MBR then chose the format.(exFAT or MSDOS) and click apply. If it still doesn't work, format the drive on the PC.
I went to format a Bootcamp / second partition on my boot drive and it's restricting my size, I know I have more free space available
Your partitions are stacked like so before creating the second partition.
OS X - say 100GB
10.7+ Recovery HD (if on 10.7+, hidden)
Your partitions are stacked like so after creating the second partition.
OS X - 50GB
Bootcamp / Second Partition - 50GB
10.7+ Recovery HD (if on 10.7+, hidden)
When you try to create a Bootcamp / Second partition it's started from the bottom free space remaining on the OS X Partition and works it's way up until it hits data written on the drive from OS X. If that partition size is too small to install Windows, then Bootcamp will say it can't continue.
Hard drives write data on a drive from the top down where it can find space. When files are deleted it leaves holes and thus the data on the drive isn't all nice and compact all the time to provide all the free space at the bottom of the drive to create the second partition.
To fix this requires cloning one's OS X partition to a external drive, hold option boot from it to check it out, erase the OS X partition in Disk Utility (booted on the clone) onto the internal drive OS X partition, then reverse clone the external OS X partition back onto the internal OS X partition.
This will effectively write all the data of the OS X partition starting from the top of the partition and working downward, so when you boot off the internal OS X partition you will be able to format your second partition with more of your remaining free space available.
Read more about cloning here
Can I have more partitions on my boot/external drive?
With OS X, your allowed to create one more partition using Bootcamp on the boot drive. It's formatted FAT32(MSDOS) which if you go and install Windows 7 via their install methods will change that format to NTFS.
You can create/delete one more partition using Disk Utility or Bootcamp on the boot drive and then use Disk Utility to change to another format for that partition, but DU won't format NTFS. You only get one more partition on the boot drive, if it's used up for Bootcamp that's it.
You can use Disk Utility to change the format of the Bootcamp partition to GUID and OS X Extended Journaled and install OS X there, or clone it from the first partition even.
Technically it's possible to have up to 16 partitions on a external drive, but the OS X internal boot drive is limited to one extra.
Use a blank external drive, Disk Utility and experiment applying Partition Map schemes, creating partitions and formatting those partitions.
Creating/deleting a partition on a booted system can be risky, always have a copy of your personal user data files off the machine before proceeding.
Bootcamp or Disk Utility hangs or won't format
If you have a problem formatting your second partition in Disk Utility or Bootcamp, quit Bootcamp, then use Disk Utility >Erase Free Space > Zero first on the OS X partition, let it work, takes a while and then try creating your Bootcamp or Disk Utility partition again.
If you have a problem formatting your external drive, or any partition on that drive, don't care about the data there, then use Disk Utility >Erase > Zero either that whole drive, or the partition (be careful what you choose!) then try formatting again. Zero Erase the entire partition or drive removes all data on it.
Trouble with transfering data or formatting?
What's the difference in Disk Utility between the entire drive/media, and partitions?
The entire media will have the size and the makers name on the left, the partitions on it are indented underneath.
When one selects their entire drive media, they are affecting change to all partitions on the drive, hidden included.
Note: In 10.7+ selecting the entire drive likely is only possible via a external boot drive/disk/USB (or via Firewire Target Disk Mode w/another Mac)
Note: 10.6 doesn't have a Recovery Partition like 10.7+ has, if it was it would be the last partition on the media.
This graphic shows a 10.6 internal drive with the secondary partition and the hidden EFI partition made visible, you won't see this normally.
Supplemental information about formats can be found on: https://discussions.apple.com/docs/DOC-3003