Why does PS CS5 (12.0.4) write to original file when I 'save as copy'?

Question

Here's the scoop: I open a PS file. I chose 'Save As' but uncheck 'Show Layers' in the dialog box. So now it's essentially a 'Save A Copy'. When the new file is saved the original file is written to (although nothing changed) and now has the same modification date as the 'Save A Copy' file. Super strange and causing issues. Because if one flattens or does anything to the original file, those changes are baked into the original file!
Any ideas? Thanks, in advance.
Mac OS 10.7.3
iMac i7 3.4GHz
32 GB RAM
256GB internal SSD (scratch disk)
1 TB internal SATA (system/data disk)
ColorEdge CG275W main monitor via Thunderbolt
Photoshop 12.0.4
Nik ColorEFX Pro 3.x
Onone Perfect Resize Pro 7.x

Answer 1

I've checked with CS5.1 and the original file isn't being written to here on OS X 10.6.8 when a copy is written. The only file that's written to is the one named in the Save As dialogue. I don't know what to suggest, sorry.
Edit: I see you have 12.0.4 - CS5. You can probably ignore my post since a fix to your problem may have been made in CS5.1.