External drive formatted ExFAT no longer mounting

I believe macOS has another update available for v14.3. I saw in another thread that some exFAT issues were fixed by the latest update, but I don't know if this issue has been resolved. If the 14.3 update does not help, then you may need to wait for another update. Unfortunately Apple rarely reveals any information about any bugs they know about & are currently trying to fix. If macOS 14.3 doesn't allow you to mount the drive, then try using Windows to repair the file system, then see how the drive behaves with macOS 14.3.

FYI, the problem most likely involves Apple changing the exFAT & FAT (ms-dos) file system drivers from a Kernel driver to a user-space driver in macOS 14.x Sonoma. This involves some major changes to the driver and it seems the Apple macOS developers didn't get it quite right. Here is a screenshot of the relevant section:

Interesting: I noticed Finder says my external HDDs formatted as ExFAT have Creation & Modified dates such as "December 31, 1969", which is completely wrong. I re-formatted them as ExFAT within the past year, some using fully-patched Windows 10 Pro, and others using Apple Disk Utility. As long as I disable Spotlight for these drives (several are 4TB each, and one is 6TB) they work perfectly. Over recent months I've copied many TBs of data to & from these drives without error.

Starting with Sonoma 14.3.1, my 128GB SD card formatted as ExFAT no longer appears in the Finder. Disk Utility recognizes it, and can Unmount it, but if I then Mount it in Disk Utility, the SD card still doesn't appear in the Finder. Same problem happens using this 128GB SD card in two different SD card readers which work with all my other SD cards. The SD cards which work for me are all 16GB and smaller.

This 128GB SD card formatted as ExFAT works perfectly in other devices such as my Windows 10 PC, video camera & stills camera

I have several multi-terabyte hard disk drives (spinning disks) formatted as ExFAT which _do_ appear in the Finder, and work perfectly -- as long as I turn off Spotlight for those disks. When Spotlight was enabled by default for all my ExFAT HDDs, they would hang/crash and even get corrupted. Disabling Spotlight for my ExFAT HDDs resolved that issue.

I don't know if Spotlight is causing the problem I see with the 128GB SD card, because I can't disable Spotlight for the SD card since it doesn't appear in the Finder.

Just tried mounting an SD card formatted with MSDOS and a USB stick formatted with ExFAT, both got mounted as soon as they were plugged into my 2018 15" Intel MBP with Sonoma 14.3.1.

The funny part is that get info says that it was created in 1970, but that's irrelevant, or maybe not, for Finder/macOS reading dates wrong. It even says that today is 01:00, January 1st, Thursday 1970.

My very old WD HDD external disk too gets mounted. It was formatted around 2010. The SD card and the USB sticks are more than 5 years old, and had been formatted with Linux then. All of them opened in earlier macOSs too.

Maybe, you should partition your WD 5TB HDD to smaller partitions in ExFAT, then try to mount them on your MacBook. They should mount.