macOS Mojave - Failure w/thumbnail cache same problem with High Sierra

A little piece of unrelated advice. Never upgrade from a current version of OS to a newer version until you have fixed any problems that you have had in the older version.

Hopefully, you won't have to go too far with the following before your problem is fixed. However if it isn't fixed then I suggest you take it in and have the service people take a look at it. Make an appointment at the Apple Genius Bar for service. If you need to find an Apple Store - Find a Store - Apple. If you did not purchase it from Apple then you will need to look to the vendor for service or How to find your nearest Apple Authorized Service Provider (AASP) or Distributor (AAD).

A Troubleshooting Procedure that may Fix Problems with macOS El Capitan or Later

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

Be sure to back up your files before proceeding if possible.

1. Shutdown the computer, wait 30 seconds, restart the computer.
2. Disconnect all third-party peripherals.
3. Resetting your Mac’s PRAM and NVRAM
4. Reset the System Management Controller (SMC)
5. Reset your Startup Disk and Sound preferences.
6. Start the computer in Safe Mode. Test in safe mode to see if the problem persists, then restart normally. Also, see Use safe mode to isolate issues with your Mac - Apple Support and Playing Safe- what does Safe mode do?.
7. Use the Apple Hardware Test to see if there is any hardware malfunction. How to invoke and interpret the Apple hardware tests - CNET.
8. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list. Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
9. Repair permissions on the Home folder: Resolve issues caused by changing the permissions of items in your home folder.
10. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button under Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
11. Download and install the OS X El Capitan 10.11.6 Combo Update or 10.12.6 Combo Update or Download macOS High Sierra 10.13.6 Combo Update as needed.
12. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
13. Erase and Install OS X Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:

1. Select Disk Utility from the Utility Menu and click on Continue button.
2. When Disk Utility loads select the drive (out-dented entry) from the Device list.
3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.
4. Set the Format type to APFS (for SSDs only) or Mac OS Extended (Journaled.)
5. Click on the Apply button, then wait for the Done button to activate and click on it.
6. Quit Disk Utility and return to the Utility Menu.
7. Select Reinstall OS X and click on the Continue button.

14. If none of the above helps then see How to Downgrade macOS High Sierra and macOS Reversion- How to Downgrade from High Sierra.

15. If you get here without success then make an appointment at the Apple Genius Bar for service. If you need to find an Apple Store - Find a Store - Apple.

Hey, appreciate you sticking with this issue enough to get not only an actual response from the support team at Apple, but also what seems like a followup?

Still amazing that this exact same issue plagued both the beta versions of High Sierra (and was reported both here and to Apple) and ended up in the shipping version of High Sierra (and was reported both here and to Apple) and then was actually addressed in an update (which was pretty cool), and now we are going through the exact same cycle in Mojave. I guess the hopeful good news is that Apple did get the message last time. The frustrating thing is they repeated the same problem, and it doesn't give me a lot of confidence for the next MacOS update.

Also, I will be the first to admit this is not a mainstream type bug-only those who use custom icons and external drives experience it. But it is absolutely a debilitating bug in terms of functionality-grinds the Finder to a halt.

So, went to downgrade lastnight, simply go into recovery mode, go into disk utility and erase the MacintoshHD drive, but, I didn't have a MacintoshHD drive, and wasn't 100% on which to remove and so on, so I went onto the live chat, and finally got it sorted late lastnight, one senior was getting me to erase each internal drive I did have but after doing the first one I accidentally clicked back on my iPhone browser and lost the chat, so I went back on, and the other senior basically got me to go into Terminal first, and create the MacintoshHD and do some other things, and I then had Fusion back with MacintoshHD under it, so everything was sorted that end, so I then went into Reinstall macOS in which it installed Sierra, and then I upgraded to High Sierra, checked my external hard drive with my custom thumbnails and it works perfectly, custom thumbnails show up instantly! i'm a bit gutted due to the fact that I work at night mostly and I loved the dark mode on Mojave :-( it was a lot better for my eyes, but I suppose i'll have to give that up if I want my custom thumbnails on external drives to work as they should!

Only problem I have now, is that on Time Machine, when I drag my files back to the fresh iMac, which were originally on my iMac (not an external drive), they have the custom thumbnails on my folders, BUT they also have an 'icon?' file in each folder that has a custom thumbnail, hundreds and hundreds! and I don't want that, but when deleting that icon? file, it removes the custom thumbnail from that folder :-( so it looks like i'll have to redo all my custom thumbnails on my iMac again!!! the ones on my external drive are fine obviously as they weren't on Time Machine, but yeah, that sucks, and I don't know how I can get rid of those 'icon?' files without it removing the custom thumbnails too :-(

Regarding my movies, I basically download the DVD cover (or poster) for that specific movie, then I use the Image2Icon app, which then enables you to create a icon that looks like an actual DVD case with the cover/poster on it, so basically when I go to my movie folder it'll have images of actual DVD cases with the cover on them instead of the usual file type icon or small player icon, once you create the icon you right click the movie, get info, and drag your icon to the icon part in the get info window, makes it look so much better, and easier to scan through too.

Also, see caching - macos Mojave beta - problem with image thumbnails

macOS 10.14 Mojave problems: how to fix them | TechRadar

Mojave 10.14 Thumbnail problem - Apple Communityhttps://discussions.apple.com › Mac OS & System Software › macOS Mojave

I still have the exact same issue as I did in High Sierra, which you've already state, was fixed with an update, and to have the exact same problem again with Mojave is so annoying!!!

I'm trying to sort out my movie files, which i've spent countless hours sorting custom thumbnails for (the actual movie covers) using the Image2Icon app, and I just opened my 'N' folder which has all my movies in beginning with N, and i'd say it's been open now about 5 minutes and it's still very very slowly putting the thumbnails on, there is about 87 files in that folder, and to take this long just to add the thumbnails to the file is pathetic!!! and also then when I click a file and go on 'get info' to add a custom thumbnail to it, I just get the spinning beachball for ages and I can't do much, spending nearly £3000 on a computer, extra hundreds for RAM, and Apple can't even get simple thumbnails to show up! it's driving me nuts.

I'm in the exact same boat guys and it's been a real pain ever since upgrading. It took me ages to even find out exactly what the issue is but now that it's listed here, at least we can relate that you're not the only ones suffering from it :)

There is no workaround it seems at all. The only way I have found to be even able to work is by not opening the folders with the actual files in them but move the whole folders and luckily, I only have 1 movie per folder so drag & dropping the folder directly into an app circumvents the loading of the thumbnails sometimes....totally idiotic and crazy to see that this has already been fixed before and is now back :(

I really hope this will get fixed once again because working like this is impossible...it's actually a problem I would always watch out for every single time before an upgrade if it comes back in a new macOS version..

Same here, had that exact issue with High Sierra when I downloaded it, thumbnails and custom thumbnails not loading, or taking ages to load! also other issues.

Upgraded to Mojave, same issue again now with the thumbnails, as well as I keep getting the loading symbol whenever I try and do anything like move a file, or open a app or load a folder, it took 16 seconds to move a movie file from my downloaded folder to my desktop with that annoying loading symbol and I couldn't do anything else at all while waiting.

I also use external drives for everything, which are only turned on when I actually need whatever it on that drive, and the iMac itself isn't low on space, I have 24GB of RAM, I have the cleaner app so I keep files etc clean.

It's getting really annoying that Apple can't release an OS without millions of people having issues, the amount people spend on their products etc, you'd think Apple would make sure everything works correctly.

I think a lot of users understand that posting in these 'discussion' forums don't equivalate a solution, but rather start a discussion to initiate and engage in conversation (regarding issues pertaining to the respective forum) in hopes to either find a solution or communicate with each other tips and/or work abouts.