macOS: Mount exFAT partitions created in Windows #

0. Issue

An 8TB USB HDD formatted as exFAT in Windows 10 (using the default settings) would not mount in macOS 12.

Disk Utility showed the exFAT partition greyed out. Right clicking it and selecting "Mount" returned:

Could not mount “8TB”. ( error 49223.)

Terminal fared no better:

   % sudo mount -t exfat /dev/disk2s2 ~/mnt
   mount_exfat: /dev/disk2s2 on /Users/user/mnt: Invalid argument
   mount: /Users/user/mnt failed with 71

1. Cause

Seagate's Disk Utility Errors and Questions for macOS 10.11 and higher explains:

Error Message - error 49223
This error message has been seen with drives formatted ExFAT. When using a drive between macOS and Windows, drives should always be formatted on macOS because not all Windows allocation unit sizes (block sizes) are supported by macOS. An unsupported allocation unit size will keep the drive from mounting. This error will be displayed when trying to mount the drive manually with Disk Utility. If you are using this drive between macOS and Windows then back up any important data on the drive and reformat the drive ExFAT on macOS.

2. Solution

Rather than reformatting, exFAT for FUSE (which supports clusters up to 32 MB) was used to successfully mount the partition in macOS:

% sudo /usr/local/sbin/mount.exfat-fuse /dev/disk2s2 ~/mnt

3. Prevention

Finding only incomplete or incorrect guidance online, I decided to test every allocation unit size displayed in the Windows format GUI:

2048 bytes
4096 bytes
8192 bytes
16 kilobytes
32 kilobytes
64 kilobytes
128 kilobytes
256 kilobytes
512 kilobytes
1024 kilobytes
2048 kilobytes
4096 kilobytes
8192 kilobytes
16384 kilobytes
32768 kilobytes

(as well as the two smaller options, 512 and 1024 bytes, offered in the CLI), prepping partitions via the command prompt, e.g., C:\>format Z: /fs:exfat /v:test /q /a:128k .

macOS was able to natively mount exFAT partitions with cluster sizes from 512 bytes to 1024 KB, but not from 2048 KB through 32768 KB.

4. Notes

/mac | Mar 04, 2022

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