You have a Mac system that is running the previous version of the Mac operating system so you decided to upgrade it to Mac OS X 10.10 Yosemite. When you tried to do this you faces OS X Yosemite installation error which results in to a non booting Mac computer. The problem is really serious as this won't let you to access the files that are stored in the Mac drive. You will have to format the drive reinstall the operating system again for OS X Yosemite installation error fix. When the operating system is working again then you must be thinking to recover the data which can be restored using time machine. When the time machine dos not work then you can use the Mac data recovery software to scan the Mac drive to get back the data. You will get the list of the recoverable files in the preview which can be selected for recovery. So you should fix OS X Yosemite installation error and download the software to recover files from Mac which will be an easy process for any kind of user.