I created a photoshop file on my pc in ver 6. then I moved that file over to my MAC which has ver 7. I open the file, modify it and try to save. It says the file is locked. I go to the Information on the file and it isn't locked. In fact you can't even lock it.
Did you happen to "move" the file via a CD-ROM from a PC with any O/S prior to XP? Not sure if this applies when going to a Mac, but when you transfer files via CD (even between two PCs with O/S prior to XP) the files are automatically Read-Only until you change it in the properties. Maybe that's waht's happening, but the Mac doesn't allow you to change the Windows set Read-Only property. Either way, a workaround might be doing a Save As on the Mac instead of trying to overwrite the original file.
I doubt this is the problem, but other cross-paltform issues to consider are that the Mac may not like a long file name that Windows has no problem with or the Mac may not like the file extension used by Windows since it doesn't typically use extensions.