Q: Snow Leopard upgrade - keeps Photoshop?

Yes.

If you are worried about losing anything when doing a major systems upgrade/update,

Before embarking on a major OS upgrade, it would be wise, advisable and very prudent if you backup your current system to an external connected and Mac formatted Flash drive OR externally connected USB, Thunderbolt or FireWire 800, Mac formatted hard drive. Then, use either OS X Time Machine app to backup your entire system to the external drive OR purchase, install and use a data cloning app, like CarbonCopyCloner or SuperDuper, to make an exact and bootable copy (clone) of your entire Mac's internal hard drive. This step is really needed in case something goes wrong with the install of the new OS or you simply do not like the new OS, you have a very easy way/procedure to return your Mac to its former working state.

Older version of CarbonCopyCloner found here.

http://mac.filehorse.com/download-carbon-copy-cloner/2734/

Older versions of SuperDuper can be found on the SuperDuper Website and Homepage.

http://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html

Then, determine if your Mac meets ALL minimum system install requirements.

Mac OS X v10.6 Snow Leopard system requirements

Purchased Installer disc here.

http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard

To install Snow Leopard for the first time, you must have a Mac with:

An Intel processor

An internal or external DVD drive, or DVD or CD Sharing

At least 1 GB of RAM (additional RAM is recommended)

A built-in display or a display connected to an Apple-supplied video card supported by your computer

At least 5 GB of disk space available, or 7 GB of disk space if you install the developer tools.

You will need to use one of the data cloning software methods to insure that all of your installed software will still function.

TIme Machine Backups, if I remember correctly,  do not preserve the actual application functionality.

The plus of using data cloning software is that you create a backup that your Mac can boot up to if your Mac's internal hard drive fails or something goes wrong with an upgrade or update.

A full system copy (clone of your system) makes a complete, fully running copy of your Mac's internal hard drive, both fully running system and all data.

When was the last time you had the drive connected up and a Tine Machine backup was done?

Is the external drive partitioned at all?

How recent is the data on that external drive?

Is any of your iTunes or iPhoto libraries on that drive?

Is any of the info on that drive important?

I have to ask this as you would need to either write over the current data with the cloning software or do a complete erase and reformat of this drive before proceeding with the cloning process.

My feeling is that the more redundancy of data you have, the better it is to have more than one source of backed up data.

You should consider purchasing another good quality external  FW800 external hard drive for the clone and use your existing as is as a Time Machine backup drive OR use it to add another cloned copy to that drive, as well or do as I did and partition each drive with two partitions.

One that is a little bigger in size for an entire system clone plus 70 GBs and use the other extra partition on each drive as strictly data storage.

Okay,

Then we CAN use that drive if you think you're fine to do so.

You can use the Disk Utility App you have on your Mac, now, to erase and reformat that external drive as an OS X Extended Format (journaled) with a GUID partition scheme. One of the tabs on the right window in Disc Utility will let you partition that drive into more than one partition.

Do an info (Command-I) by clicking once on your iMac's hard drive icon to determine how much space your entire system and all the stored data there takes up.

As a buffer, you can add, at least, another (50 GBs) to that total (I,usually, add an additonal 75 GBs additonal space, but I recommend, at least, adding another 50 GBs) to the Total space needed to create for the first partition on the external drive.

What ever is left on the second partition is for just data storage.

Make sure to name the external drive's partitions so you know what they are when you see them and to have a name for the boot partition that includes the version of OS X that is on that bootable partition.

CarbonCopyCloner is best as it has many other features, but it is more expensive.

SuperDuper will work too, but I believe it is a cheaper cloning app than CCC.

Either way, you will use the cloning app to make a bootable copy of your Mac's internal hard drive to the first partition of your connected external hard drive.

Before proceeding with the upgrade to OS X Snow Leopard, test the external clone by picking the cloned partition on the external hard drive for startup in OS X Preferences Panel under the Startup Disk icon and click the restart button and wait to see if the Mac boots to the external drive (this will take a little longer than usual as you are booting to an external source).

Once booted to the external drive, run in this mode and do some things to confirm everything is working fine.

Then in the OS X Preferences Panel, choose the Startup disc icon, again, to pick your Mac's internal hard drive to boot back to and click the restart button to have your Mac boot back to its internal hard drive where you can begin the OS X 10.6 upgrade process.

For future info.

It is very desirable to have more the one external backup drive as drives can and do fail and having redundant drives with duplicate data is a way to protect your data over the long term.

I have three external drives with different bootable systems on them, but my saved data partitions are identical on all of these drives.

"When I went to the link you posted for CCC it says the download is freeware?"

That version of CarbonCopyCloner (that is still compatible with your version of OS X) maybe free or Donationware before the developer decided to actually charge a fixed price for the software. CCC used to be free to download and use with the caveat that, at some pointm users were supposed to contribute a small Donationware fee. Obviously, the developer was not getting many donations for his efforts and decided,  in later update versions, to charge what he feels is a fair price for software that almost every knowledgeable Mac users uses.

"I don't think I'd need to partition the external hard drive or add more GB.

My computer says:

Capacity: 297.77 GB

Available: 231.66 GB

My La Cie has 500GB, so I'd only be using a small portion of it......  right?

Nina"

Yes, you have only used 67 GBs on your internal drive.

Therefore, I would create two partitions on the external drive with the first partition ( the one that will get the bootable clone) set it size at 120 GBs for a partition size (70 GBs+50 GBs=120 GBs) and the second partition will be your remaining storage on the external drive.

The extra space on the boot partition is there for when the system gets larger and you need the room to clone it to the external drive, again.

OS X 10.6 Snow Leopard maybe a larger OS X version and you may need that extra space on the boot partition for cloning, later.

After you have had a successful upgrade to Snow Leopard and you have completed the myriad of updates to get all of your OS X version to OS X 10.6.8, updated to all of the various security and Apple software updates, you are going to clone your entire system, again with the new 10.6.8 once you know that everything is working properly on your Mac's internal hard drive.

I feel it is more space efficient to have any OS version on its own separate partition.

I always partition my Macs internal and external drive to have as small a boot/startup partition as possible and the rest of the drive as storage.