There are only a few HD makers out there. I suggest in the 1tb range checkout either WD, Hitachi and Seagate. While your current HD installs are 2 adding a 2nd SATA is very possible, its the power requirement that maybe a limitinmg factor. It may work for awhile but constant power demands will tax your PSU. I suggest you also upgrade the PSU to a higher wattage unit. If you can add the new HD, great if not then consider removing the old 80gb after you swap/clone data to it for install replacement. I suspect the 80gb is your boot drive. Also, you need to check how your bios setup boots there maybe a legacy IDE or similar wording in order to select SATA boot. Or chuck it all for a few $ more, get a 2tb or 3tb drive instead. replace all drives with 1 big SATA one and enjoy the speed increase and all.
tada -----Willy 
I have an old computer with an Asus K8N motherboard, currently it has a 80GB IDE hdd and a 500GB SATA hdd. I need more storage and I'm thiking of buying a 1TB HDD.
What is the best 1TB HDD for this motherboard? I've searched that the hdd needs do be SATA I.
As the motherboard already has 2 hdd's do I need to unplug one hdd to install the new one?
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