I Have a mid-2010 (MC511LL) 27 inch iMac. it started going slow then, when turned on gets up to the progress bar, 3 quarters along, then just goes to a black screen with the spinning rainbow circle and no further. Now by all play and searches this is a hard drive failure BUT I had the harddrive removed and it passed WD (Western digital testing) and I was told hMars drive is not faulty BUT operating system/bootsector may be. That being the case, if the harddrive is fine - how do I reinstall operating system or get this working considering a cant get it to boot up? I did when the drive was in try every single type of reset holding down various keys for safe mode etc etc now worked. any help would be appreciated!!! I also have no access to a bootable drive or another Mac. I have a 21 inch Mac that within 3 days did exactly the same thing but turns itself off after the progress bar so 2 dead iMacs! Help please..