Time Machine and the Lightroom Cache

You’re asking about several different things, they have different answers and locations.

Lightroom Classic maintains these files, among others:

Catalog database

Previews

Smart Previews

Camera Raw cache

XMP metadata sidecar files

Does Lightroom store the (allocated) local cache in '~/Library/Caches'?

By @Another Madala

If you literally mean Lightroom (not Lightroom Classic), then the local cache is in the Pictures folder by default, and that path is displayed in Lightroom Preferences / Cache as shown in the picture below. There’s a Browse button in case you want to move that to another volume with more free space, such as an external SSD.

Lightroom Classic has multiple caches. For the Develop module only, it caches at the Camera Raw Cache location specified in Preferences / Performance. You control that location and size limit, as shown in the picture below. Note that below that, there is also a Video Cache with a size limit, always stored at:

~/Users/[username]/Library/Caches/Adobe/Lightroom/Video

For all modules other than Develop, what they cache are the previews and Smart Previews, not the Camera Raw cache. Those are always located in the same folder that contains a catalog.

The question on the catalog was not the Lightroom Classic concept, but rather a reference to where Lightroom stores the local index.

By @Another Madala

If you mean the “‘local index” as in the record of what images are being tracked and the folder path to each image, for Lightroom that is stored in the cloud and cached locally inside the Lightroom Library.lrlibrary file. Technically, that’s really stored as a macOS Package file, which is a file that contains other files (different than a folder). It can be opened and inspected if you right-click it and choose Show Package Contents, but never alter anything in that package or things could get corrupted. (This is exactly the same way Apple Photos works; the Apple file Photos Library.photoslibrary is also a macOS Package and its contents must also never be tampered with.)

For Lightroom Classic, that’s what the catalog does (the catalog is the index), with the help of the other files in the same folder that contains that catalog.

In the case of failure or transfer to a new machine, is it better to let Lightroom rebuild this from the cloud than to force restore the old version?

By @Another Madala

For Lightroom, yes, it’s best to rebuild from the cloud simply because it’s cloud-based. All you have to do is install the Adobe Creative Cloud desktop app, sign in, and install Lightroom. After Lightroom sees that you are signed into your Creative Cloud account, it syncs down your photo info from the cloud to the local cache location you saw in Lightroom Preferences above. Originals will be downloaded on demand.

For Lightroom Classic, as I said before, a local catalog cannot be rebuilt from cloud data. For one thing, the cloud data knows nothing about what local folders the images were in, so those can’t be rebuilt. Any successful restoration depends on how well a local catalog is backed up, because it has to be restored from that backup. 

Last question. Where does Lightroom store the sidecar edit files? I am planning to keep a local copy on an external hard drive, but my understanding is that this is the RAW only.

By @Another Madala

Any time XMP sidecar files are used, they must always be stored in the same folder as the raw file, with the same base filename as the raw file. If a file format can be written to, such as a DNG, TIFF, or JPEG, then metadata can be written into the file itself so no XMP sidecar file is created.

Lightroom does not use XMP sidecar files for any cloud-stored originals (photos you edit using its Cloud tab). Instead it stores the data in the cloud and syncs it to the local cache. Lightroom does use XMP sidecar files for any images you edit using its Local tab, similar to if you edit a raw file with Adobe Camera Raw. There is that strict separation between the Cloud and Local tabs, they work very differently. The Local tab doesn’t interact with the cloud data in any way.

Lightroom Classic does not use XMP sidecars file by default, the catalog is primary storage for all edits. To have Lightroom Classic generate XMP sidecar files, you either have to do it manually by selecting photos and then choosing the command Metadata > Save Metadata to File, or you can have them written automatically by enabling the option Automatically Write Changes Into XMP (in Catalog Settings, Metadata tab).