Mavericks - Hard Drive Space
The requirement for free hard drive space to upgrade to OS X Mavericks is the same as was required to upgrade to Mountain Lion. Your machine must have 8GB of available space to be able to download and install Mavericks. Historically, Apple has been increasing the amount of hard drive space required with each OS version they release, but they have been able to keep the requirements for Mavericks the same as Mountain Lion - good news for users who are running tight on hard drive space!
External Storage Options from Ramjet
If you don't have enough hard drive space free or have just enough to upgrade, it might be time to consider replacing your existing hard drive with a larger drive, or purchasing an external hard drive to use as additional storage for your files. In the time where many of us store our family photos, videos, and entire music libraries on our computers, it's easy to see how drive space could be used quickly.
Apple's Time Machine
Luckily, if you decide to replace your main hard drive, Apple makes it easy to migrate your entire machine onto the new drive using an external hard drive, Time Machine, and the "restore from a backup" function. Alternatively, when you upgrade to Mavericks can also be a great time to install to a new, fresh drive and simply bring all your data files over rather than cloning the entire system and system files.
Read on to find out what else we recommend before upgrading to Mavericks.