Changing the hard drive in your Mac computer from the main hard drive to the slave drive may seem like an intimidating task, but it’s actually quite simple and much safer than changing it in other types of computers. Follow these instructions to change your main drive to a slave. This method also works when you’re trying to install another operating system onto your Mac or when you have an older computer that’s not as powerful as it once was and you want to start over by installing a fresh operating system on a new hard drive, in addition to making the second hard drive your main drive. How to configure a secondary hard drive on your Mac
Which OS X version do you have?
Your version of OS X will determine what methods you can use. For example, in Lion and Mountain Lion, Apple introduced an easier way to set up multiple drives by using Target Disk Mode (TDM). If you have Mavericks or Yosemite, TDM won’t work—instead, you’ll need to use one of two other methods.
What should you backup first
Whether you’re just starting out with backups or are looking for ways to improve your current plan, it’s important to know where you stand. A popular method of determining whether you have enough backup protection is performing a simple test: pick one folder and make a copy of it every day for two weeks. If anything were to happen (fire, theft, flooding), would those files be gone forever? If so, then there’s probably something missing from your backup strategy.
Create a new bootable partition
The most convenient way to add a new partition is with disk utility, included with every copy of Mac OS X. To launch it, open up any Finder window and hit Command+Shift+G. In the box that appears, type in disk utility (or alternatively open Applications > Utilities > Disk Utility) and click Go. When you see Disk Utility listed as one of your options, click on it.
Delete all partitions from the external HD
To begin, plug in your external hard drive. Then open up Disk Utility and look for it under External. Select it and click Erase. Finally, confirm that you want to delete all partitions from it (the disk will be entirely blank), then click Erase again. It takes some time, but when it’s done go ahead and close Disk Utility and reboot as usual.
Re-create the partitions from your old computer
If you want to copy everything from your old hard drive (including your operating system, all of your files, and all of your settings), you can’t just throw it in as another disk. You’ll have to re-create all of its partitions first.
Copy everything back from backup onto new partition
Time Machine backs up all of your important files and data, but you’ll need to copy those files back onto your new partition. Apple will walk you through that process if you need help. Use Time Machine: It’s possible, though not recommended, to just drag and drop files from backup onto your new partition.