Summary of this discussion on Adobe groups.
So you render preview files in Premiere and the next time you launch the program you’re met with red bars in the sequence – indicating the need to re-render (even though those render files are still sitting on the hard drive right where Premiere last left them). While Adobe is purportedly aware of the issue, it’s still pretty annoying.
Premiere seems to lose track of rendered preview files on the sequence that was displayed at the time the project was saved before exiting. When Premiere is re-launched, by default, it opens to the last displayed/saved sequence.
1. Create an empty sequence. Leave it empty. Rename it if you like (“Select me before saving and exiting”) – or not.
2. Work in your other sequence(s). Render your previews as needed.
3. Before exiting Premiere (this is the important part) switch to the empty sequence and save the project. Then exit.
4. The next time you start Premiere: a) Select the project, b) Wait till all media is loaded (Premiere reports this at bottom). It will open to the empty sequence. When you switch to the work sequences, you should have green bars where the previews were previously rendered.
When you launch Premiere to a sequence that has rendered previews, Premiere immediately checks to see if the render files are available. If it doesn’t “see” them in those first milliseconds, they are ignored and you get a red bar. (In old versions of Premiere, if renders weren’t found when/where expected you were given an opportunity to locate them). The problem is, if you have a lot of media in a project, it can take several seconds for everything to load and the render files are last. By then it’s too late. Premiere didn’t “see” the files in question, so they were ignored for that sequence.
This is also the reason why the problem seems to be hit and miss. If you have a small project, all the media including the render files load quickly before Premiere has a chance to “ignore” what it thinks might be missing in the way of rendered previews.
From Premiere manual
Ensure that Adobe video applications use the same cached files
Adobe video applications can automatically insert a unique document ID into each imported file. These unique IDs ensure that each application accesses the same cached previews and conformed audio files, preventing additional rendering and conforming. In the Media section of the Preferences dialog box (or the Metadata section for Soundbooth), select Write XMP IDs To Files On Import.
This setting is global—a change in one Adobe video application affects all the others. This setting also results in new file modification dates when IDs are initially inserted.
To save rendering time when transferring a project to another computer, move both cached and original files.