Don't Run Upgrade from App Store app!

Updated: 
07/31/2015

Understand the difference between Upgrade and Update

  • Upgrade:OS X 10.9.8 → OS X 10.10
  • Update:OS X 10.10 → OS X 10.10.1

OS Upgrade means major version upgrade, i.e., OS X 10.9 (Mavericks) → OS X 10.10 (Yosemite) You do not want to kick Upgrade run fro App Store Upgrade Notice.

Even if you obtain an installer, upgrading over existing older version of OS is not recommended (by me)

Instead, it is much safer to create an empty space (volume or disk), then run a fresh OS install, then use Migration Assistant to move your previous data to your spanking brand new OS

2 ways to upgrade OS X safely

  • Buy a new internal hard drive, run a fresh OS X install, followed by moving data from the previous OS X using Migration Assistant app
  • Employ 2 partition system (see this article), reformat the volume where 2 OS X version was installed, run a fresh OS S install, using Migration Assistant all move the data from the other partition where the previous OS X is still available.
  • I employ both above
    • I buy new internal hard drive every time a new OS X version is released
    • Partition the new internal drive into 3 (see this article), copy 2 volumes from the previous hard drive
    • Install the new OS X onto the 3rd partition
    • Using Migration Assistant app to move the data from volume where the previous OS X is copied to the new OS X volume