If you are trying to use an installer you downloaded previously…
OS X installers downloaded prior to 14 February 2016 won’t work- See TidBITS article: Previously Downloaded OS X Installers No Longer Work [Added 02 Mar 2016]
Trying to update a client’s computer today. After downloading Yosemite installer it failed to run. It would start but fail with the following error:
This copy of the Install OS X Yosemite application can’t be verified. It may have been corrupted or tampered with during downloading.
First step was to download again. That failed. Tried downloading again. Failed. Started looking for help and found many suggestions that worked for some users. After trying reset date, repair permissions, repair drive, remove RAM and downloading multiple times, user purchased a USB thumb drive and we were able to create a bootable OS 10.10 Yosemite installer drive using the installer that failed to run.
- The Install Mac OS X Yosemite.app must be in Applications.
- Format the USB drive as GUID in Disk Utility. 8GB or larger
- Name “Untitled”
- Paste the following into the Terminal
sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Yosemite.app --nointeraction
- Enter Password
- When the process is finished you will see Copy Complete. Done. (the process took about 30 min so be patient)
- Select the drive in System Preferences > Startup Disk to install.
Thanks to Dan Frakes for instructions on creating USB installer. For more details, see Dan’s blog:
[Added info: December 3, 2015] Check out the FAQs on DiskMaker X page for help with errors making a boot disk. DiskMaker X is a free app that many find useful in creating a bootable install disk. This can be very helpful if you are running El Capitan and want to install an older version of OS X like Yosemite on another drive.