If you installed Yosemite…then had an installation issue…didn't have a backup…and couldn't access your HD…you wouldn't be asking this question!
Then you would know the answer to the "necessary" question!
But if you had a backup…you would sure be glad you had it!!!
Statistically speaking…it is on the rare side to have a major install issue with an OS upgrade where the HD (and files on it) would be inaccessible. But if you had your "LIFE" (so to speak) on the computer:
- MS Office files
- music library
- videos
- family photos
- PhD. dissertation
- Novel you are writing
- etc.
And you had an install issue & lost all of this stuff (no backup)…you would be one UNHAPPY PUPPY!!!
So it is almost always advised to make a backup of the HD before an OS upgrade. AND besides that…everyone should have "normal" backups anyway (just in case of an HD, SSD, or flash storage failure). This can happen at ANY TIME without warning!
Heck…just an unexpected power spike…or a thunderstorm strike can fry the computer & the storage inside.
Of course if someone does not store ANYTHING very important on the computer…then there's not a whole lot to be concerned about.
- Nick