TL;DR version
For Mac OS X to read-write exFAT formatted HDD, two options

Flash Drive Allocation Unit Size 256gb Mac Os X
Flash Drive Allocation Unit Size 256gb Mac Osx
Flash Drive Allocation Unit Size 256gb Mac Os Catalina

Plug in your external HDD to the USB port of your computer then go to my computer find your external drive right-click it and select format. A new window will open, select the desired file system, keep the allocation unit size to default and check mark quick format (or it will take more time). And now this exFAT works on both Mac and Windows 10. Edit: I just figure out why in my case. When formatting exFAT in Windows 10 from Disk management. If formats the exFAT using Allocation unit size of 2048 kilobytes. If you right click on the drive and reformat the exFAT using allocation 1024 kilobytes. Then it will work in. Apple Mac mini - Intel Core i3 - 8GB Memory - 256GB SSD Intel® Core™ i3 8th Generation Quad-core processor8GB 2666MHz DDR4, 256 GB SSDFour Thunderbolt 3 (USB-C) portsGigabit Ethernet and 802.11ac Wi-Fi Model # MXNF2LL/A.

Format using Mac:
1. Disk Utility ->
2. Erase ->
3. choose exFAT ->
4. OK
Format using Windows:
1. My Computer ->
2. Right Click HDD ->
3. Format ->
4. choose exFAT ->
5. AUS 128 kilobytes->
6. Start

Background
Turns out exFAT exists so that Mac and Windows can co-exist harmoniously. There are many forum discussions and how-toarticlesoutthererecommending exFAT if you want to share files between Mac and Windows.

What most of them failed to mention is the correct allocation unit size / cluster size necessary for the harmonious relationship to work. Most of them talked about what the allocation unit size does, which might be misleading for the purpose of getting it up and running seamlessly.

My Story
I want to use an external HDD as the scratch disk for a video editing project on a Mac OS X 10.7.5. But my files are on my Windows 8.1 laptop HDD, which was formatted in NTFS. By default, Mac OS X can only read but not write to NTFS HDD. Of course there are NTFS read-write solutions out there for Mac. Among them Tuxera NTFS, Paragon NTFS, or NTFS-3G FUSE.

Flash Drive Allocation Unit Size 256gb Mac Os X

Disclaimer: I have not tried Tuxera or Paragon. I used NTFS-3G FUSE from 2009-2013 on my Macbook Snow Leopard, so far so good. I have not tried it on later versions of Mac OS X. There is a high chance of it not working on OS X 10.7 and later (see Known Issues).

Flash Drive Allocation Unit Size 256gb Mac Osx

Then I found out that if I format the HDD in exFAT, it should work for both Windows and OS X. I figured that since most of my files are videos with BIG file sizes, I chose 4096 kilobytes for the AUS. It formatted nicely. But lo and behold, OS X doesn’t even recognize the HDD. I tried to force mount it but nope, it doesn’t work.

So I figured that if I use OS X’s Disk Utility to format, it should work for both Mac and Windows. This time it works! Turns out Disk Utility formatted the HDD with 128 kilobytes AUS (131072 bytes divide by 1024).

Just for kicks, I used Windows to format it again with AUS 128 kilobytes. Yeap, it works.

Flash Drive Allocation Unit Size 256gb Mac Os Catalina

Conclusion
128 kilobytes is the harmonious constant between Windows and Mac OS X 10.7.5. Use 128 kilobytes AUS while formatting a HDD so that it works for both Windows and Mac OS X.