I use a PC and use LR5. I have a catalog on an external drive, I got a message "LR has encountered an error while reading the preview cache and needs to close". I could work later looking for some backup, now I do not get to see the photos comes a exclamation mark. I searched the lost photos automatically and can not find, I can find manually in the same external drive, I have to look 16000 photos ... anyone can help find a solution. Once you have met? Thanks!
The easy fix to the preview cache problem is to delete the preview cache folder in the same folder as your catalog. LR will recreate the previews as needed.
For finding the missing files, first try to find any missing folders.
In the Library module, in the Folders panel, right-click and "show parent" as needed to make the tree look like what it looks like on your hard drive.
For any folders marked as missing, find those first. This should fix most of the missing photo problems.
For any photos still missing, you will need find those individually.