Q: An error occurred while extracting files from the package "Essentials.pkg"

Please try again after taking each of the following steps that you haven't already taken.

Step 1

Reset your computer’s PRAM.

Step 2

If your model has user-replaceable memory, and you've upgraded the memory modules, reinstall the original memory and see whether there's any improvement. Be careful not to touch the gold contacts. Clean them with a mild solvent such as rubbing alcohol. Aftermarket memory must exactly match the technical specifications of the machine.

Step 3

Back up all data to at least two different storage devices, if you haven't already done so. One backup is not enough to be safe. The backups can be made with Time Machine or with Disk Utility. Preferably both.

Erase and install OS X. This operation will destroy all data on the startup volume, so you had be better be sure of the backups.

Step 4

Make a "Genius" appointment at an Apple Store, or go to another authorized service provider to have the machine tested.