> Checking in the cache

I have lost an old photo, which was on my previous Mac. I have its last Time Machine backup on a hard disk, however can’t find it there. Could I check the QuickLook Cache on that, perhaps?

In High Sierra and earlier, it may be possible to recover old images from a startup disk even when the original has been lost or damaged. QuickLook, which is responsible for creating Finder thumbnails and previews, generates these and stores them as files inside a cache. By inspecting the contents of that folder, you may be lucky enough to find the image that’s gone missing.

The cache is located in the hidden folder /var/folders/t9/[long ID]/C/com.apple. QuickLook.thumbnailcache where [long ID] is a seemingly random jumble of characters, but only on a startup disk. There are two problems which make this of limited value.

It was recognized as a security vulnerability over two years ago, and Apple responded by locking it down to make the cache essentially inaccessible in Mojave. So if that system was upgraded to macOS 10.14 or later, you’ll no longer be able to open it.

That folder’s contents are considered by macOS to be ephemeral, so they aren’t backed up by Time Machine either. Neither is the old QuickLook cache carried across during migration from an old system to a new one. Cloning the disk using a third–party utility like Carbon Copy Cloner should make a copy of it. Unless there’s a normal backup copy on the old hard disk, you won’t find it there.