error on imac? I have an iMac & the latest version of iTunes, I have burned many CDs using this same medium, and with this latest version of iTunes without a problem, but now I can only burn a few songs before a loud noise, rather like grinding, starts and eventually the error message: "Attempt to burn a disc failed. Burn failed due to a medium write error." pops up. and the disk ejects itself Is it a software or a hardware problem? please help

I would appreciate any help, as I have a lot of discs to burn, and soon.