Formatting Disks on a Mac

From time to time one of the members will post a question ‘how do I format this disk on my new Mac?’  or ‘will this model of disk work on my Mac’ and there seems to be some confusion around disk management for your computers.  Hopefully this post will clarify a few things for you and make life a little easier.

Firstly there is no such thing as a Mac-specific disk or a PC specific disk.   Under the covers they are all identical and it’s only the branding and packaging that will change to appeal to a different demographic.  Both Macs and PCs support Thunderbolt and the various flavours of USB as these are industry standards.  As is the disk inside the case at the other end of the cable, whether it be SSD or HDD.  They are all the same.  Okay there might be some difference with some of the chips that control the inner workings of different products, but you will not be aware of this and while this mattered in the early days of SSD some 10+ years ago it’s irrelevant now.   SSDs have matured to the point where they are both affordable and reliable and brand differences aren’t as important as the warranty or the price.  

Vendors such as OWC target their products at Mac customers, but their products will also work on PCs, you just have to buy them without the Mac-specific tools and use the native Windows tools instead.  The only exception to this is Drobo who now only support Mac users and you need a special piece of software to access their disk appliances and this is no longer available for PC users.  All other disks are pretty much universal and can be used on any computer.

So why can’t that new disk I’ve just bought be used in my Mac?   It has probably been formatted to use on a PC. What does that mean?  Before you can write data to your disk the operating system has to carve it up into little pieces so when it writes data to it, it understands where everything is – a bit like a yellow pages, but stored in a way the computer understands.  PCs and Macs use a different method, or format, to achieve this and for the most part they aren’t compatible.  The table below shows the three most common disk formats used on external disks for PCs and 

What usually happens is, you buy a disk and it is formatted as NTFS.  You plug this into your Mac and while you can read the ‘readme’ file on there and open the various tools that come with it, you can’t write any data to the disk.  This is because Macs can only read NTFS disks, they cannot write to them.   Any disks that have been branded as Mac-specific will be formatted for Mac OS and PCs will not be able to read them.  The only format that is interchangeable is EXFAT.  This however is a very old file system that is an update of the one used for MS-DOS on PCs over 40 years ago.  Use it for temporary file transfers between Mac and PC by all means, but don’t rely on it for primary storage as file corruptions are common on EXFAT.  It’s there for convenience not reliability.  

So how do I make this new disk work on my Mac?  You need to format it, and you do that by using the Disk Utility tool on your Mac.  See the steps below.

1.  I have a 1TB disk that has been formatted for use on a PC and has some data stored on there already.   I know this data is no longer needed so I am okay to format this disk.

2. I open Disk Utility to format the disk.  The quickest way is to use Command+Space to enter Spotlight and search for it.  As we can see it is a 1TB USB disk that is formatted as NTFS.

3. I selected the correct disk from the list on the left, and then clicked on the ‘Erase’ button on the top of the window.

4. A this point I can rename my disk if I want to, and I can select the format I want to use for the disk from the drop-down menu.  I changed it from NTFS to Mac OS Journaled.

6.  I’m happy with my selection so I proceed with the disk format by clicking on the ‘Erase’ button on the bottom right of the pop-up window.

7.  My disk has been formatted as a Mac OS disk and now my Mac will be able to read and write data to it.

This disk can now only be used on Macs.  If at a later date you decide you want it to be used on both Mac and PC, then reformat it again and choose EXFAT as the format type.  Just make sure you copy any data you want to keep off the disk before doing so.   

You can’t break a disk by formatting it, well you can if you use some very low-level tools that bypass the operating system and talk directly to the disk, but most normal people don’t do stuff like that.  Only Geeks, and only special Geeks at that.  So use your disk as you need to and remember it’s just a tool that you use with your computer.  Obviously don’t throw it around, drop it or get it wet and it will probably last a very long time.