Just bought a used iMac, need help.

Before you do anything you need to perform a firmware upgrade so you can perform network recovery: Computers that can be upgraded to use OS X Internet Recovery. Select the appropriate link to download the update for your model. After that has been done:

Install OS X Using Internet Recovery

Boot to the Internet Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND-OPTION- R keys until a globe appears on the screen. Wait patiently - 15-20 minutes - until the Recovery main menu appears.

Partition and Format the hard drive:

1. Select Disk Utility from the main menu and click on the Continue button.
2. After DU loads select your newly installed hard drive (this is the out-dented entry with the mfgr.'s ID and size) from the left side list. Click on the Partition tab in the DU main window.
3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the format type to Mac OS Extended (Journaled.) Click on the Partition button and wait until the process has completed. Quit DU and return to the main menu.

Reinstall OS X: Select Reinstall OS X and click on the Install button. Be sure to select the correct drive to use if you have more than one.

Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.

This should restore the version of OS X originally pre-installed on the computer.

A mid-2011 may have come with Lion preinstalled. You can check by restarting and hold the Command+Option+R keys (you need a good internet connection) and the machine will restart on the Apple servers if it came with Lion preinstalled. You can then use the Utilities drop down menu to erase the hard drive if you want and reinstall Mac OS X.

If the system is now asking for the previous owner's Apple ID and password, that means that person installed the current Mac OS X and you will not be able to update or do anything since it is tied to that persons ID.

Is there a reason to run the AHT? Do you suspect something is physically wrong with the machine? The AHT may find things that are grossly wrong, but the subtle stuff just doesn't get picked up. For that you need the Apple genius bar techs to run their diagnostic suite.

Does the iMac boot up and work? Does it connect to the internet? If so, the first order of business would be to restart and hold the Command+Option+R keys to see if you will restart on the Apple server. Then you can erase the hard drive and install the original Mac OS X, most likely Lion. From that point you/your parents will own the iMac and you can decide if you want to upgrade Mac OS X as you think appropriate for your parents.

If the Apple server is telling you it needs the previous owner's Apple ID and password to download Lion, then it sounds like that iMac came with Snow Leopard installed and the previous owner upgraded to Lion.

You can see if it came with Snow Leopard (requires the DVD for installation) by calling 1-800-MY-APPLE and give them the serial number of the iMac and they can send you replacement for the install DVD, for a fee of about $20. You can install the Snow Leopard system, go to Software Update to make sure you are updated all the way to 10.6.8, and then go to the newer OS X version of choice.

If they tell you the machine came with Lion, we have to drop back and figure out why the server is not accessing the installer as it should, the original installer is not tied to an Apple ID or password.

That is great - what Mac OS X did they install? Was it Snow Leopard or Lion?

Curious to understand the message the Apple server was giving you.

tjk6277 wrote:

OS X Lion. The genius that was helping me said it shipped within a couple days of a new release so it could have been set up with either Snow Leopard or Lion.

Which was one of my points in an earlier post - I have one of the MBPs from 2011 that came with Lion (which I wanted at the time) and had to make sure the Apple store pulled a later production run machine so it did not have Snow Leopard. That was an issue in figuring out why your iMac would not load Lion from the Apple server without an Apple ID and password - the preinstalled OS X should have been downloaded without doing that. Or so Apple keeps telling us. Guess you were the unlucky one to hit a glitch in the system.

At least you now have the operating system installed and fully own the machine. So the question becomes, do you stay with Lion or go to Yosemite? All depends on the needs of your parents. If they do not need, nor have any interest in those features of Yosemite then Lion is not a bad choice for them. Gives access to the Mac App Store and to iCloud. The things lacking are iMessage and iBooks, are those important? A matter for their consideration.

Anyway, congratulations on getting this resolved easily at the Apple store - those are some good folks.