It will not be erased, but accidents can happen so it behooves you to make a backup before proceeding.
I recommend that you follow this outline:
How to Install OS X Updates Successfully
A. Repair the Hard Drive and Permissions:
Boot from your current OS X Installer disc. After the installer loads select your language and click on the Continue button. Then select Disk Utility from the Utilities menu. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the installer. Now restart normally.
If DU reports errors it cannot fix, then you will need Disk Warrior (4.0 for Tiger) and/or TechTool Pro (4.5.2 for Tiger) to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.
B. Make a Bootable Backup Using Restore Option of Disk Utility:
1. Open Disk Utility from the Utilities folder.
2. Select the destination volume from the left side list.
3. Click on the Restore tab in the DU main window.
4. Check the box labeled Erase destination.
5. Select the destination volume from the left side list and drag it to the
Destination entry field.
6. Select the source volume from the left side list and drag it to the
Source entry field.
7. Double-check you got it right, then click on the Restore button.
Destination means the external backup drive. Source means the internal
C. Important: Please read before installing:
1. If you have a FireWire hard drive connected, disconnect it before installing the
update unless you will boot from this drive and install the update on it.
Reconnect it and turn it back on after installation is complete and you've
2. You may experience unexpected results if you have installed third-party system
software modifications, or if you have modified the operating system through
other means. (This does not apply to normal application software installation.)
3. The installation process should not be interrupted. If a power outage or other
interruption occurs during installation, use the standalone installer (see below)
from Apple Downloads to update. While the installation is in progress do not use
D. To upgrade:
Purchase the Snow Leopard Retail DVD.
You must purchase Snow Leopard through the Apple Store:
Mac OS X 10.6 Snow Leopard - Apple Store (U.S.).The price is $19.99 plus tax. You will be
sent physical media by mail after placing your order.
After you install Snow Leopard you will have to download and install the
Mac OS X 10.6.8 Update Combo v1.1 to update Snow Leopard to 10.6.8 and give you access to the
App Store. Access to the App Store enables you to download Mountain Lion if your computer meets
Snow Leopard General Requirements
1. Mac computer with an Intel processor
2. 1GB of memory
3. 5GB of available disk space
4. DVD drive for installation
5. Some features require a compatible Internet service provider;
fees may apply.
6. Some features require Apple’s MobileMe service; fees and
Boot From The OS X Installer Disc:
1. Insert OS X Installer Disc into the optical drive.
2. Restart the computer.
3. Immediately after the chime press and hold down the "C" key.
4. Release the key when the spinning gear below the dark gray Apple
5. Wait for installer to finish loading.
6. Follow instructions.
E. If updating:
1. Download and install update(s)
2. Use Software Update, or
3. Download standalone updater(s).
For some people Time Machine will be more than adequate. Time Machine is part of OS X. There are two components:
1. A Time Machine preferences panel as part of System Preferences;
2. A Time Machine application located in the Applications folder. It is used to manage backups
and to restore backups. Time Machine requires a backup drive that is at least twice the capacity
of the drive being backed up.
Alternatively, get an external drive at least equal in size to the internal hard drive and make (and maintain) a bootable clone/backup. You can make a bootable clone using the Restore option of Disk Utility. You can also make and maintain clones with good backup software. My personal recommendations are (order is not significant):
2. Data Backup
3. Deja Vu
5. Synk Pro
Visit The XLab FAQs and read the FAQ on backup and restore. Also read How to Back Up and Restore Your Files. For help with using Time Machine visit Pondini's Time Machine FAQ for help with all things Time Machine.
Although you can buy a complete external drive system, you can also put one together if you are so inclined. It's relatively easy and only requires a Phillips head screwdriver (typically.) You can purchase hard drives separately. This gives you an opportunity to shop for the best prices on a hard drive of your choice. Reliable brands include Seagate, Hitachi, Western Digital, Toshiba, and Fujitsu. You can find reviews and benchmarks on many drives at Storage Review.
Enclosures for FireWire and USB are readily available. You can find only FireWire enclosures, only USB enclosures, and enclosures that feature multiple ports. I would stress getting enclosures that use the Oxford chipsets especially for Firewire drives (911, 921, 922, for example.) You can find enclosures at places such as;
1. Cool Drives
All you need do is remove a case cover, mount the hard drive in the enclosure and connect the cables, then re-attach the case cover. Usually the only tool required is a small or medium Phillips screwdriver.
Kappy is trying to encourage you to protect your data, as it is far more valuable than you realize until it is gone (speaking from experience )
The upgrade will NOT touch your personal data.
However, anytime you make a major change like this, something unpredicted could occur, and having a backup is one way to protect yourself. Also from time to time, individuals do not like the upgrade and want to go back, but unless they have a backup, it is extremely difficult to go back.
Finally, you should be backing up your data on a regular bases, as computer are mechanical devices and they break, are subject to loss, theft, natural desasters, and from sometimes ourselves . Having a backup is one way to protect yourself and all the valuable information on your computer.
I am assuming you have a Mac OS X 10.6 Snow Leopard installation DVD. If so, a backup and install is not all that difficult.
Buy USB external disk at BestBuy (or other), plug it into iMac, use Applications -> Utilities -> Disk Utility to format as HFS+, use System Preferences -> Time Machine to select the disk for backup, wait for backup to finish.
Then insert Snow Leopard DVD, double click on the installer, answer the opening questions then sit back and wait.
The install is rather easy. The backup a little more effort.
If you do not already have the Snow Leopard DVD, you need to get that first. :)