Cannot boot iMac.

I have a mid 2010 27" iMac. I keep it up to date, incl. the latest OS, but recently it won't boot. It either grinds/white screen or I get the ?/folder. I know what these are & tried all startup shortcuts. I can boot to the OS disk (Snow Leopard), but don't want to reinstall the OS as I would prefer to recover some data. It's not life/death, I won't lose much that I don't have backed up, but there are some things I would like to recover. I try opening the disk utility from the OS installer menu, but it just runs and runs and runs 'gathering disk information'. How long should I reasonably wait for this? If I reinstall the operating system, does it wipe the disk and all my data with it, or does it partition the drive leaving the data recoverable? Is there an option to partition it so I can recover data? If I have to open it up and remove the harddrive to attach it and recover files that way, I will, as I do have many other systems, so the unit/hardware isn't a huge loss to me if I can't put it all back together again. Though I am reasonably technically competent/experienced. Help? Thanks.