ALL Smart Previews Vanished from LR5

Worked "offline" yesterday afternoon editting some of the thousands of Smart Previews that were created in the past couple of weeks. My final action was removing about 70 of them from one folder that contained 400 images. Upon opening LR today, not one Smart Preview is shown for the raw images on external drive or the images on hard drive. What's happened. Are they auto-deleted after a specified period? Can I get them back from backups? Thanks.
Running on macbook pro with El Capitan.

If you choose to backup when you close the program, you might be able to restore a prior catalog backup...

Several points occur to me:
First,

Smart Previews are not "in" catalogs. They are on the HD in their own folder and are low resolution (2560 pixels) .dngs.

No, they are not auto-deleted and yes, you can create or recreate them at any time. As a matter of fact the only way to get rid of them is by using the command from the Library menu and right next to it is the command to build SPs, so it is a bit hard to understand how you couldn't know that - if you did actually discard them, as you believe.

skid00skid00 wrote in post #17892155
If you choose to backup when you close the program, you might be able to restore a prior catalog backup...

tzalman wrote in post #17892771
Several points occur to me:
First,

Smart Previews are not "in" catalogs. They are on the HD in their own folder and are low resolution (2560 pixels) .dngs.

Whether an SP exists for a given photo original and the SP's identifying tag are the subjects of a note in the catalog and that way LR knows to use it when the original is not available, but the mere presence of that note in a restored backup catalog will not cause the recreation of an SP that has been deleted.


No, they are not auto-deleted and yes, you can create or recreate them at any time. As a matter of fact the only way to get rid of them is by using the command from the Library menu and right next to it is the command to build SPs, so it is a bit hard to understand how you couldn't know that - if you did actually discard them, as you believe.

Of course you could through Finder/Explorer manually delete the SPs, but why would you, since they are not identified by the names of the original files with which they are associated, only a code number, and you would be striking blindly?

BTW, the SP is never used when the full sized file is present, although its existence is indicated by the icon below the histogram.

I believe it is a mistake to make SPs during import for every photo - unless you are aware that they have absolutely no function other than acting as surrogates for off-line originals and the rest of the time they are simply inertly occupying disc space and when you have no further need for them you transfer the edits to the originals and discard the SPs. It makes more sense to build them if and when needed (unless at the time of import you already know that you will be doing at least some of the editing away from where the Raw files are stored.)

An individual SP, being low res and highly compressed, is not big, but I can remember one case when a LR user had accumulated a fine collection of more than 50,000 Smart Previews, almost all of which related to images that he hadn't viewed in years and for which he never really needed SPs.

I appreciate you both taking time to reply. Apparently, I wasn't clear in my post. To be clear, I'm seeking help (i.e. instructions) restoring (without re-building) the thousands of smart previews that I created just a week prior for all my "keeper" photos. (These photos are stored on an external drive.) I also need to know why they disappeared so that I can prevent this from happening in the future.

I have to admit that you have me somewhat confused. I don't understand why you have all those backups. You only need to keep the last three or four. Moreover, having the backup catalogs on the same drive as your active catalog is tantamount to having no backup - but this is beside the point and doesn't address your question. My guess is that the Smart Previews are still there, you just aren't looking in the right place. You seem to be looking in the backup folders (those dated folders) and they contain only copies of the catalog file (.lrcat) because that is the only thing that LR backs up. If you open a backup .lrcat as though it were an original .lrcat catalog file, you will force it to automatically create a new regular previews folder inside the dated folder and alongside that backup .lrcat file, but not a Smart Previews folder - for that you have to give the "Build Smart Previews" command. And because there are no SPs in the dated folder and because a catalog can only relate to an SP folder that sits next to it (in this case, in the dated folder) that backup catalog doesn't indicate any SPs.

But when you went up one level to the Lightroom folder, it does contain, in addition to the Backup folder, the SP folder and your original catalog. Open that catalog (the only one you should be opening since the other copies are only for storing away somewhere, to be used only in an emergency) and it will relate to whatever SPs are in the folder that was first created in 2014 but has presumably been added to since then. Open that Smart Previews.lrdata folder and you will see it contains a multitude of subfolders. At 12.5 GB it probably contains well over 6,000 SPs

Thanks. I found the solution to my "issue". I simply had to tell LR to view an earlier catalog and the previews appears using, File - Open Recent command. I don't understand how these different catalogs get created or why, but will certainly have to learn and understand.
My LR backups are stored on my hard drive. My hard drive gets backed up routinely on an external drive. I do see that I can delete all those older LR backups. Not something I learned from the LR tutorials.

Since you have another more inclusive backup routine that is independent of LR, I would just go into Edit>Catalog Settings>General and turn off LR's backup.