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Solved: USB-C Samsung T9 External SSD (exFAT) doesn't mount in macOS Sonoma

TL;DR: you'll need a Windows machine. Plug in the SSD and a "repair drive" popup will show up. It takes 2 seconds, now it also works on mac again.

____

Problem


I've been editing a lot of 8K video, and I have a pair of Samsung T9 external SSD (4TB) to extend my 2023 MacBook Pro M3 Max's 1TB of onboard storage. I kept them formatted as exFAT, which is the default they ship with.

Now before we go further, if like me you only work with macOS and don't need Windows interoperability, do yourself a favor and format your empty drives to APFS.


Today I plugged in my nearly-full (300 GB free out of 4TB) Samsung T9 to my MBP with the provided fast USB C cable.

The SSD blinked blue and then the light shut off, but in Finder my drive was nowhere to be found. I tried different cables, different ports, I rebooted my machine, to no avail. I tried plugging in another (empty) Samsung T9, and this one worked.

If it were a hardware problem, it would blink red.


I took to Google to find solutions, which includes this forum: Unable to access External Drives and Disk… - Apple Community


Investigation


1. Samsung Magician: don't waste your time installing this utility, it's useless.


2. Waiting: some people suggest waiting half an hour+. doesn't help.


3. Disk Utility & First Aid: Applications > Other > Disk Utility

After a few seconds, my volume appears there, unmounted and greyed out.

This means the system knows a USB external drive is connected to it, it can fetch info about it (name, size etc), but it cannot connect its file system.


There's a First Aid button, but it fails:

Performing fsck_exfat -n -x /dev/rdisk4s2
Checking volume
...
Checking active bitmap
The volume SAMSUNG T9 could not be verified completely
File system check exit code is 12
Restoring the original state found as unmounted
Error: -69845: File system verify or repair failed
Underlying error: 12


Shows there's corruption somewhere, and macOS is unable to fix it.


4. Command Line: same thing as the Disk Utility app, in the terminal.


First thing to do is getting the identifier for the volume.

$ diskutil list
...
/dev/disk4 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *4.0 TB     disk4
   1:         Microsoft Reserved                         16.8 MB    disk4s1
   2:       Microsoft Basic Data SAMSUNG T9              4.0 TB     disk4s2


I recognize the name "SAMSUNG T9" which is what my volume is called. The identifier is disk4s2 (most likely different for you).

Try to mount manually:


$ diskutil mount /dev/disk4s2
Volume on disk4s2 failed to mount
If you think the volume is supported but damaged, try the "readOnly" option

$ diskutil mount readOnly /dev/disk4s2
Volume on disk4s2 failed to mount
If you think the volume is supported but damaged, try the "readOnly" option


no dice


$ diskutil verifyVolume disk4s2
Started file system verification on disk4s2 (SAMSUNG T9)
...
Error: -69845: File system verify or repair failed
Underlying error: 12

$ diskutil repairVolume disk4s2
(same output)


No dice. Notice how the error message is the same as the one in Disk Utility's First Aid.


$ fsck_exfat -d /dev/disk4s2
* Checking volume.
** Checking main boot region.
7814002688 total sectors; 512 bytes per sector
...
Checking bitmap cluster 5
--- [1] Evicted 1 buffers (131072 bytes; 8 pages)
Read      offset = 0x000007660000  length = 0x020000
Checking bitmap cluster 6
0 clusters were marked used, but not referenced
0 clusters were marked used and CLUST_BAD
0 clusters were marked free, but referenced
** The volume SAMSUNG T9 could not be verified completely.


We don't learn much but the evicted buffer is a hint of some minor data being corrupted. Otherwise the files on the SSD are listed correctly, and it doesn't seem like there are a lot of issues.


5. Windows


Since the SSD is not fried, I plugged it to my Windows machine. A pop-up showed up immediately, saying that the drive had an issue and could be repaired. It took no time at all to repair it, and then I could see all my files again! Plugging it back to the MBP, it works again.


Suspected Root Cause


It looks like macOS Sonoma on Apple Silicon shutting down or going to sleep with a large exFAT drive plugged to it, especially if files have been edited recently or the SSD is almost full, something must bork, and macOS cannot fix it.


So the solution should be one of the four:

  1. Do not use exFAT as it's buggy on macOS, use APFS instead.
  2. Have a Windows machine handy at all times to repair the drive when it borks.
  3. Have a Parallels VM on your mac with Windows installed to run the Windows repair utility (need USB to be passed through to the VM). I haven't tried that one.
  4. If none of those things is an option, take extra care to "eject" your drive as soon as you don't use it any more. However this solution will only reduce the probability of an issue.

MacBook Pro 14″, macOS 14.5

Posted on Jul 23, 2024 2:18 PM

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Solved: USB-C Samsung T9 External SSD (exFAT) doesn't mount in macOS Sonoma

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