Create a second partition on a Macbook disk without losing data

Is it possible to create a new partition on a disk that we are already using, containing data, and without losing them?
If that is possible using Disk Utility, tool already included in Mac OS X. In the following example, we will create a second partition on an external USB hard drive.
And what does it mean to do this? For example, avoid using a second hard disk (and the corresponding USB connection on your computer) for Time Machine, since it tends to occupy all of the available space, or to clone the boot disk in a second partition that we can format without affecting the rest.

Make it clear that this is “invasive surgery” and is not without risks. It is always convenient to use “First Aid” (also part of “Disk Utility”) to verify the correct state of the disc. And remember that a mechanical failure would affect the integrity of the data of both partitions.

Once we open “Disk Utility” (Applications / Utilities) we will see on the left the list of disks connected to the Mac. In most cases we will see only the “boot disk” (with the operating system) and perhaps the disk that we use for “Time Machine”, although in the screenshots we will also see the disk “USB_300” that we want to “partition”.
Note that each disk is represented as “the disk itself” and a kind of “subfolder” or “subfolders” that correspond to the existing partitions on each disk. At first, we will see only a single partition per unit, but once we finish, we will see how the USB disk goes to show two “subfolders”.

Now, first, WHAT WE SHOULD NOT DO. We should not select the disk, click on the “tab” or “partitions” and change, in “Volume Scheme” to “2 partitions”, because yes, we would have two partitions, but having FORMAT the whole disk and losing all data stored in it.

ON THE CONTRARY, we must click on the “+” icon at the bottom to ADD and create a new partition but preserving the existing partition and the data stored on it.
Of course, there should be enough free space on the disk to allocate it to the new partition. That is, if we have 80GB free, this is the maximum size that could be allocated to the new partition. But in fact, we must leave a margin. If there are 80GB free we should not make a new partition of more than 60 or 70 GB. Otherwise, the partition with the data would be left almost without space and there could be problems.
As we said before, once we finish we will see how the USB disk goes to show two “subfolders” or partitions, and we will see how in the Desktop appear mounted two “units of disc”.


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