Extremely slow export as catalog, >24 Hrs; LRClassic

Thanks to all who replied, I figured it out. I had selected "build smart previews", I think that was the problem because when I cancelled build smart previews during the export it zipped right through the remaining export, which was about 40% left. 

Exporting as a catalog with negative files, all the photo image files associated with that catalog will also be exported (copied) to the external drive. Same with the smart previews as you mentioned.

USB 3 seemed like hot stuff back in the day but now it pales in speed compared to USB-C and Thunderbolt connections. I would say the time for the export is about right.

You don't explain why you are exporting everything, but if it's for travel or temporary projects, like GoldingD said, you could start with a blank catalog, add the new images to that and when finished export only the new stuff in a catalog that can be imported into your principle catalog.

A simple copy of a similar size catalog, previews and photos to an external drive that I recently did took 5 hours on a USB 3 connection.

The source catalog, a few issues to look at (not necessarily a full list)

• Any folders missing per the Folder Panel?
• Any Photos missing? (mind you a folder could be missing, causing that)
• Any apparent duplicate folders
• Any indication that you have the Capitalization error in the Folders panel(kind of relates to the prior)
• Any Folders in the Folders Panel that do not actually exist (might have no photos, and as such might not scream out look at me, I am missing)
• Any Hard Drives in the Folders panel that have no business being listed.
• Have you Optimized the catalog lately
• Have you run an Integrity check on the catalog lately.
• Is the catalog getting corrupted? Anything else misbehaving?
• Is any form of Sync running in the catalog, Face Detection, Address Lookup, Folder SYNC, Sync to the Cloud.
• This one fairly far fetched, but in the current Import Screen, is the Destination valid
• Both hard drives when checked do not indicate issues correct?
• Both hard drives are external. So, any issue with the cable (USB?) connecting them to the computer? Any issue with another device (like a Android device, a typical issue). Any conflicts (USB). Connected directly to computer, not a hub?
• Does your Anti-virus get in the way (log files in the Anti-Virus would show that). Proper Anti-Virus would not do this.
• I see this is a laptop. How is it's cooling. Any dust Bunny's blocking the vents. Is it overheating.

Oh, one to add

• If you look at the destination, what is currently being written? Perhaps those previews? (Lots of small files)

For both drives, how much free space in percent. And I do mean percent. Looking for 20%, some say 25% or better.

I don't know why this is taking so long, but a suggestion would be to copy the catalog file, negatives and smart previews using your operating system.