What's the minimum amount of free drive space that I need? My Mac is starting to operate slowly, taking a long time to boot or to launch an application. It also seems unstable, sometimes giving me the rainbow cursor for very long periods of times, even locking up completely.
Do I need a bigger hard drive?
There are many different types of problems that can manifest the symptoms you describe. Insufficient RAM or even hardware failure could be the culprit. But one of the most common causes of the problems you describe is not having enough free space on a startup drive.
Filling your hard drive until it's almost full is fraught with issues. First, your Mac needs some free space for creating swap space to manage memory use. Even when you have adequate RAM, OS X will reserve some space at startup for memory swap space. In addition, individual applications usually use some disk space for temporary storage.
The point is that many pieces of the OS and many applications use hard drive space, usually without your being aware of it. When it gets your attention, it's usually because of erratic system performance.
In general, you should keep as much of your drive free as possible. If I had to put a minimum on the amount, I would say keep at least 15% of your startup drive free at all times; more is better. If you're getting to the point where you worry about your hard drive's free space, it's probably time to either spring for a larger hard drive or archive some of the data and get it off the drive.
How did I come up with 15% as a bare minimum?
I picked this value so that some basic OS X maintenance scripts will have sufficient free drive space to run. This includes OS X's built-in disk defragmentation system, memory swap space, and enough space to create cache and temp files when OS X starts up, while still leaving room for basic applications, such as email and web browsers, to use free space as needed.
Free Up Disk Space
To free up disk space, start by selecting a target location for offloading data. You can copy files to another hard drive, burn them to CDs or DVDs, or in some cases, simply delete the files. I always look to my Downloads folder first, because it tends to collect a lot of files and I tend to forget to delete them as I go along. After that, I check my Documents folder for old and outdated files. Do I really need to store my 7-year-old tax files on my Mac? Nope. Next I look at my Pictures, Movies, and Music folders. Any duplicates in there? There always seems to be.
Once I go through my home folder and all of its sub-folders, I check the available free space. If I'm not above the minimum, then it's time to consider additional storage options, either a larger hard drive or an additional drive, probably an external drive for storing data files.
If you add more storage, don't forget to factor in enough backup storage to cover your new capacity.
Having free hard drive space well above the 15% minimum is a good idea. The minimum only ensures that your Mac will start up, operate, and be able to run a basic application or two. It doesn't guarantee your Mac or the applications will run well, or that your graphics, audio mixing, or video production applications will have enough scratch space to be function.