openSUSE LiveCDs are designed to be used without making any changes to the harddrive. Only and only if you decide to install them will they make any permanent changes to your system.
Well I’ve only tested the GNOME and KDE myself (3.5+4.2) and they all work fine.
The tools in Yast are accessible without having to give a root password. If you use them to manipulate partitions on the hard drive/s, those manipulations are real – so don’t do that. Also in a console window you can easily get rootly powers and do damage if you use some file-altering or filesystem-manipulating commands. So don’t do that either.
If you really want to be safe, and you’re comfortable taking your PC’s cover off, you can unplug your hard drive.
No reason the liveCD won’t boot without it.
You honestly don’t need to bother though - the CDs must have been used safely by millions. Don’t click on buttons that say things like ‘Delete Partition’ in ‘Expert Partitioner’ and you’ll be absolutely fine - the only part that presents the slightest risk is the installation, if you choose to install, and that will normally go fine if you tell it to do what you want it to do. Ask in the forum if you aren’t sure - and backup your important data first.
Regarding the permanent hard drives: It’s safer than windows which automatically mounts all drives in writeable mode. In Linux you will have no access to the hard drives that are permanently attached to the motherboard when you first boot. But you can mount them if you take certain conscious steps and they will be like all mounted hard drives then – accessible.
Regarding portable drives like USB drives or USB sticks (whatever): they will mount automatically if you plug them in after you’ve booted. Just like in windows. And the FAT32 pen drives will be writeable. Treat them the same way you would treat them in windows.
So, just like in windows, don’t do anything silly. You wouldn’t right-click a file in windows and select to delete it. Well don’t do that in Linux either. There’s nothing mystical about it. Just don’t cruise in the filesystem and delete files. Also, in windows you would stay away from the program called “cmd”, the command prompt. Well stay away from the command prompt in Linux too on your first couple of boots.
It’s a normal operating system, just like windows – there are no hidden traps, just don’t click any buttons that are labelled “delete”.