You probably need to check your hard drive to see if it needs repairs. Press the power button on the back to turn the machine off. Boot holding Command+R to boot into the Recovery HD, click on Disk Utility, select Macintosh HD and click on 'repair disk'. If it makes any changes click repair disk again until you get a clean pass. Restart as normal from the Apple menu. More details here:
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Hold the Power button of the iMac to turn it off. Then, start your Mac pressing Command and R keys while your Mac is starting to start into OS X Recovery.
Then, open Disk Utility, select Macintosh HD on the sidebar and verify the disk. If there's a problem, repair the disk.
- If the disk has been repaired successfully > restart your Mac and try to start OS X again.
- If the disk has been repaired but OS X doesn't start > press Command and R keys while your Mac is starting and reinstall OS X.
- If the disk couldn't be repaired > take the computer to an Apple Store to get your hard drive replaced.
As I see that you have a Late 2009 27-inch iMac, some iMacs bought between October 2009 and July 2011 have got 1 TB Seagate drives that are defective, so Apple is replacing them for free. Check if you are eligible > http://www.apple.com/support/imac-harddrive If you are eligible, take the Mac to an Apple Store to get your hard drive replaced
I would hold the power button until it powers off. I would then power it up but hold the command-option-R keys to boot into recovery mode. From there, I would select disk utility and repair my hard drive and then repair the permission. Pay attention to the SMART status of the hard drive when you select it. Make sure it says verified.
After that, I would try and boot it again.
I did not read this on time, and went to restart pressing command + r keys and carried on reinstalling osX. ( I missed the "repair disk" part. It downloaded the OS, and installed. It is still doing this action , and the time remaining is: -53 and counting...
Should I just let it carry on for some more time?
Leave it for some time (let's say, 15 minutes), and if the bar doesn't progress, turn off the Mac using the Power button.
Then, press Command and R keys while your Mac is starting and try repairing the disk. Also, check if your Mac is eligible to have a new hard drive using the last link of my first comment