Unit Allocation Size
When I connect an exfat drive formatted on a PC, it refused to mount on the Mac. After some experimentation, it turns out to be a unit allocation issue. Mac Big Sur can only mount drives with a unit allocation size of 1024K or less. If your drive is formatted with unit allocation larger than that, it will fail to mount.
This is because on larger drives, the Mac will expand a larger Unit allocation, which may be different than the default on a PC. For a 5 Tb drive, Macs will default to 256K. In fact, Macs will not mount the 5 Tb drive if it's less than 128K unit allocation and greater than 1024K unit allocation. If you format it on the PC, make sure that you format it with allocation between 128K and 1024K. Make sure that the drive mounts on both both machines.
Just to clarify what unit allocation mean, it's the bytes per sector x sectors per cluster. For example 512 byte per sector x 8 sectors per cluster = 4096 byte or 4K.
The take away is that if you want a exfat external drive that works with both Mac and PC, make sure you format it on the Mac or set the unit allocation size to between 128K and 1024K."
https://www.reddit.com/r/MacOS/comments/p7ii0i/what_i_learn_about_exfat_and_allocation_unit_size/