Creating a blank disk image for storage
You can use Disk Utility to create a blank disk image to store files. Normally, you need to gather all the files you want to be included into a single location before you create a disk image. However, with a blank disk image, you can add files to the image at any time.
To create a blank disk image:
Choose File > New > Blank Disk Image.
Type a name for the disk image, and choose where you want to save it from the pop-up menu.
In Volume name, enter the name for the volume that appears on your desktop when you double-click the disk image.
Choose the size of the disk image from the Volume Size pop-up menu.
To require a password to open the disk image, choose an encryption scheme from Encryption.
Choose “Single partition - Apple Partition Map” from the Partitions pop-up menu.
Choose “sparse bundle disk image” from the Image Format pop-up menu.
Click Create.