Mac Hacks (3 page)

When
you partition a drive in OS X, the operating system will
treat that single drive physical drive as multiple drives. The
partitions will look like completely separate disks in the Finder and
you can treat them as different disks for everyday computing.

Partitioning your drive is useful for a variety of reasons.
Partitions can help with organization, troubleshooting, and backup
planning. But there’s one big caveat: partitions won’t help you if the
hard drive dies—all the partitions go down with it.

Note: This hack requires a Windows install disk. But it
doesn’t
require a never-used install disk. I used
an old Vista install disk, but you can get away with using any disk
someone has laying around. You won’t be running Windows anyway!

It
will likely occur to you that tricking Boot Camp Assistant
is one way to partition your hard drive, but probably an unnecessary
way. After all, if Boot Camp Assistant can nondestructively partition
your disk, you can probably do it yourself. You’re right—you can do it
without Boot Camp Assistant, though you’ll need to spend some time
with Terminal.

Fire up Terminal (it’s in your Utilities folder) and you’ll see a
window that looks remarkably like an old-style computer terminal. If you
feel a little intimidated by Terminal, check out
Chapter 6
before you start; if not, it’s time to start
typing. Take
a deep breath and type:

diskutil list

Not too hard, right? What
that command does is tell your Mac to reveal all the
partitions on the drives mounted on your Mac (“mounted” doesn’t
necessarily mean physically mounted—it just means currently available to
your machine; you can have disks plugged in that aren’t mounted). The
output will vary from machine to machine, but it’s generally almost
humanly understandable. When I ran
diskutil list
on
my iMac in Knoxville, Tennessee, I got an output that looked like
this:

What can we learn from this output? SIZE, as you’ve guessed is the
size of the disk. IDENTIFIER is how the computer tells one disk from
another, and TYPE NAME is how you can tell what each disk is. In this
particular case, we have a 1.0 TB disk (disk0) that has been divided
into seven partitions. The first partition is EFI (disk0s1), which
contains some device drivers and other necessary things for your Mac to
boot; you won’t need to mess with that partition for the purposes of
this hack. I created the second partition (disk0s2) for storing movies,
so we’ll leave that one alone, too. The next one (disk0s3) is a recovery
disk generated during the install of Lion or Mountain Lion. We won’t be
altering that partition in this hack either, so we can skip Apple_Boot
Recovery HD.

This brings us to the disk we want to repartition: Apple_HFS the
300 (a.k.a. disk0s4). Before we partition the disk, we should decide
what size partitions we want. To figure that out, we need to know how
much of the disk’s space is already in use (we don’t want to lose any
data, now do we?). To find that out, at the Terminal prompt,
type:

df -h

The
df
command tells your
Mac to list information based on file systems, and the
–h
flag tells Terminal to list the data in
human-readable form. The output will tell you more than you really want
to know so, in this case, we’ll just pay attention to the disk we’re
going to partition. Here’s the relevant information:

Well that is
almost
humanly understandable!
If you look closely, you can tell that the disk known as “the 300” is
272 GB in size and that 68 GB have been used, leaving us with 204 GB
available. That’s certainly enough space to make it worth our while to
create a new partition.

Let’s
see what the limits of our partitions will be using the
diskutil
command:

diskutil resizeVolume disk0s4 limits

Which tells us:

For device disk0s4 the 300:
Current size: 292.2 GB (292227104768 Bytes)
Minimum size: 78.3 GB (78349324288 Bytes)
Maximum size: 292.2 GB (292227104768 Bytes)

Those are the limits of what we can do; we’re free to choose any
value within those limits. In this case we’ll use 100 GB for the old
disk (“the 300”) and 190 for the new disk (which will be named,
unimaginatively, “NEWPART”). To get the new partition made, we need to
use
diskutil
to tell the program exactly what we want
to do. To do all that, type:

diskutil resizevolume /dev/disk0s4 100G JHFS+ "NEWPART" 190G

That doesn’t make a much sense if you just look at it. Here’s what
each part of that command represents:

You’ll need to adjust the command to your specific desires. Once
you get all that typed into Terminal (careful—spelling and spacing
matters), hit Return and
diskutil
will start the
process. You won’t see the nifty OS X progress bar that you’re familiar
with; instead you’ll see the Terminal version of a progress bar. (What
does a progress bar look like in the Terminal? It looks like
[***----]
, with
*
representing the completed parts.) The
process can take some time (don’t close that Terminal window!), but once
it’s done you’ll have all your data
and
a new
partition! When it’s finished,
diskutil
will print
out a list of the disks on your Mac, which includes the new
partition:

You’re probably wondering why the partition came out as 192.1 GB
instead of 190 GB. The reason is actually kind of interesting: the
maximum partition is 292.2 GB but the partitions requested by the
Terminal command add up to 290 GB. So instead of wasting that space,
diskutil
adds it on to the larger partition. (What
happened to the missing .1 GB? The world may never
know.)