Yesterday, I plugged in a flash drive to copy a folder of exported photos from my external hard drive which is the location of all Lightroom photos. When I went back to Lightroom, all my photos were listed as being resident on the Flash Drive, but they aren't there. They are still on my external hard drive. The Lightroom Catalog thinks they are on a non existent drive.
I thought about just importing them from the Hard Drive, but when I do it shows most of them as already in the catalog. What do I do to fix this?
You don't say what your operating system is, but if it's Windows, plugging in a drive can cause other drives to have different letters than they previously had. Is that what your are seeing in your operating system?
If that is what you are seeing, you just have to change the drive letters back to what they were previously.