Model A1312 / Mid 2010 / 3.2 GHz Core i3 or 2.8 & 3.6 GHz Core i5 or 2.93 GHz Core i7, ID iMac11,3

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iMac powers off during startup

A month ago, I bought  a 27" iMac Mid 2010 (MC510LL/A) with  2.3 GHz i3, 8GB DDR3SRAM, 1TB HDD Serial  ATA II, 7200 RPM,  OS-X 10,12, ATI  Radeon HD5670 Graphics.

Amazon sold me an Amazon Basics USB keyboard for it which has no Command or Option keys, but does have  Ctrl, Alt, Fn, and Windows logo keys plus a key with an icon with horizontal lines inside a square, like a list icon.

The iMac ran fine for a couple weeks, then began slowing down. Startup became very slow.  Now it will power on, sound the musical chord, and show an Apple logo above a horizontal progress indicator bar.  I hear maybe a fan. Hard drive heads  seem to  chatter for a couple seconds, then make two clicks in rapid succession, repeated about every three seconds for a few times until the progress bar reaches its midpoint and the iMac goes silent. 

I don't need to recover any files. I have no recovery or system CD, and no other Apple devices except two of the original iPads.

I would be grateful to anyone who can tell me what is wrong with this iMac or what I can do.

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Do you have a friend with a Mac? If you do you'll need to create a bootable OS installer USB thumb drive to test your system. Here's a guide to create it: How to create a bootable macOS Sierra installer drive


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Sounds like a failing hard drive to me. Try starting up from the repair partition and run Disk Utilities on the main drive by holding down the Shift key on startup.

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@mayer - This system may not have a hidden partition. It depends on what OS it's running (Lion or older) & the drive may not be stable.


@danj he stated he was running 10.12


@mayer - Oops I missed that.


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Fannyllfly will be eternally grateful.
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