Answer to: Which internal hard drive do I need?The "NVidia MCP79 AHCI" is actually the SATA controller chip on the motherboard. Your hard drive is a "ST31000528ASQ" which means that you have a Seagate 3.5" drive. If you Google "ST31000528ASQ" you'll find that some iMac owners are hearing abnormal noises from this Seagate drive. You need a compatible Seagate backup/replacement drive since you must stay with the same brand so the hard drive temperature sensor works properly. If the sensor doesn't work, the hard drive fan may run too fast (making an annoying loud noise), or the fan may run too slowly to properly cool the drive. For more details on this issue and a list of hard drives which are known to be compatible with this new method of sensing hard drive temperature, see this page: http://blog.macsales.com/2751-proprietary-cable-can-put-the-brakes-on-upgrading-late-09-iMacs The other factory-installed hard drive commonly seen in these late 2009 iMacs is the "WDC WD1001FALS" which is a Western Digital Caviar Black 1TB drive.