Updating the question because I know more about the cause of the issue, but still don't know how to solve it.
Currently I can see thumbnails for some CR2 files in windows explorer. Maybe only 1-5% of all of them. I can force windows to generate a thumbnail for any specific thumbnail (by repeatedly clicking it and pressing F5 to refresh its folder). Therefore, I don't think the problem is that I lack an app that can read and create CR2 thumbnails. I definitely have an app that can do this (FastPictureViewer). I have hundreds of thumbnails successfully generated. The problem is that I have tens of thousands of RAW files.
Just FYI, Canon's own Raw Codec, as well as Microsoft's Camera Codec Pack, as well as SageThumbs, are all based on an older version of the CR2 format so they don't work to make thumbnails for RAW files created by a newer camera, in my case a Rebel T5.
I am 99% convinced the problem is because Windows has a limit to the size of the thumbnail cache. A registry entry exists to increase this limit:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\ Create a new key called Max Cached Icons and give it a value that represents the size of if the icon cache, i.e. 4096 for 4 megs.
Here's where it gets muddy - I have gotten much conflicting info about this key.
• Some say this key should be of the type "string"
• Some say it should be of the type "Dword"
• One user suggests it should be "Qword" on a 64 bit system.
• One user says it used to be "String" but this is considered unorthodox to control with a string value, and should be a dword, and one of microsoft's updates for windows7 changes it to the more approprite dword type.
• I've seen the key both with and without spaces (e.g. MaxCachedIcons) Without is more typical for a registry key but several users go out of their way to say "yes, include the spaces even though that's unusual".
Additionally - there is no single file for the thumbnail cache, there are multiple files for the multiple icon sizes. These live in C:\Users[Username]\AppData\Local\Microsoft\Windows\Explorer and are named
So it's unclear if this setting limits the size of all 4 files together, or allows each of these 4 files to grow to the limit you set.
After deleting these and allowing windows to rebuild them, I either don't have the registry key in the correct format (though I experimented with many formats), or Windows has a hard limit on the thumbnail cache that can't be exceeded regardless of the size in the registry key. I tried 2048, 4096, 8192. Weirdly, the files at one point wouldn't go beyond 3148 kb (3 megs). Currently they are stuck at 4096 but the registry key is set for 8192. I have never seen any of these files grow beyond 4096.
I know I am on the right track because if I delete them and then start forcing thumbnails to generate using the method described earlier, the size of the icon database list grows (thumbcache_idx.db) but the other .db files refuse to grow any more. This link describes how the thumbnails all work: http://www.identatron.co.uk/wp-content/uploads/2012/01/Cyberforensics-2011.pdf
And these files definitely control the CR2 thumbnails because I can use a small app called thumbcacheviewer (or thumbcache_viewer) to open the DB files and view the thumbnails inside of them, and they are my CR2 thumbnails.
So right now, I am convinced I have hit the limit of thumbnails in windows. Now it's just a matter of... if I enter this registry key in a different way, can I bypass the limit? Or is it just set in stone?