Can I revert from 10.7.5 to Snow Leopard without problems?

I recently bought a used 21" iMac, (2009)  with Lion 10.7.5 installed, no discs.  

Unfortunately, most of my Apple software, ie Photoshop is not compatible.

I have Snow Leopard 10.6.3  which I picked up from a Computer fair a few weeks back - will it be ok to install this, so my software is compatible?  I know it's kinda backward move, but I'm not in a position to shell out for new versions of my software.

I also need to know if the Snow Leopard disc I have WILL install ok.  Apparently, some versions out there are ONLY suitable for the machine they were installed in.  
ie  - how do I know if the version I have will install ok, - how can i tell?  I have the box and disc..

Thanks all...\

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If the disk is black, it is a full commercial release which should install on any computer that shipped with 10.6.3 or older. If the disk is grey, it is an OEM disk that will install only on the model it shipped with.

If you can boot from the disk, you can probably re-install from it. Might not be a bad idea to back up your hard drive first.
Gerwin Jansen, EE MVETopic Advisor Commented:
The 21,5" iMac (2009) was shipped with 10.6.1 - Apple says "Do not use a Mac OS X version earlier than the one included with the computer." - reference: HT1159 - yours is 10.6.3.
stokesydelAuthor Commented:
Strung - I hear what you say . It's  not black.. it's a light grey, and there's NOTHING on the box that says "for installation only  on the computer it was purchased with"  or similar.  Which seems odd.  Not to mention confusing, for simpletons like met..

Backing up is awkward at the moment.. I need to reformat my backup discs.

So, if what you say is correct, Strung, the disc I have is useless?

Gerwinjansen.. thanks for that info... but it seems like the disc I have is no use at all. To me.  

Do I have to buy a new Snow Leopard from Apple to resolve this?

Thanks again.

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Probably the grey disk will not work. You may be able to find a black 10.6.3 disk on eBay. There are several full retail versions on offer:
Scott Fell, EE MVEDeveloper & EE ModeratorCommented:
What version of photoshop do you have?  I have cs3 and have used it with each upgrade from 10.5 to 10.8.   If you make sure you have ALL updates,there should be no problem.

If you have a way older version of PS... that may be another story.
R. Andrew KoffronCommented:
find a copy of snow leopard upgrade, you can install it to a thumb drive and install(if you can't find a CD). iMac came with a valid license, your totally legal to buy an upgrade and use it(costs like 20ish). only problem will be finding a copy of older OS.

The Upgrade will support a full installation and the iMac will be all the "upgrade" verification you need. there's never been any issues I've seen running full installs from upgrade media.

You'll need to sort out your backup drives and backup each users home folders. and the installs for any apps you want.

You Might have to format the drive to install the Older OS, that I'm not sure of, I always backup and format/install when changing OS, I would especially recommend it going backwards. but you could try it. (I wouldn't bother trying to install over the newer.

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You could also just replace the internal disk with a new one and try the installation on that. If it works, then fine, if not, you can go back to the original disk.
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