I can't write to my flash drive, and it is already in the MS-DOS (FAT) Format. Why?

Question

This is my first question on this website. I am using a Mac Air, and I have a flash drive in which I can read from. I am able to copy files from this onto my Desktop, or anywhere on my computer for that matter. I cannot write to my flash drive, however. I know this is a common problem for some people. I looked into their solutions. There is one difference between the cases I've found, and my case. The flash drive's format is normally a 'NT' format, which is a Window format from my understanding.

My flash drive is formatted in 'MS-DOS (FAT)' according to Disk Utility and the 'Get Info' window from Finder. It did tell me I had removed it wrongly before. I am not sure how to fix the flash drive. Do I reformat it? If so, how and what do I format it to. Please let me know.

Thank you in advance for your help, ~Rane

Screenshot:

I do have the flash drive selected in the Disk Utility picture.

Answer 1

If you don't need to use this USB disk on a Windows machine, format using Mac OS Extended (Journaled). Should you need to exchange files with Windows machines, then Format as you have with MS-DOS (FAT). ExFAT should work as well.

It is not outside the bounds of possibility that the drive itself has gotten zapped somehow and is damaged, thus not operating correctly. Copy all the files from the flash drive to a folder, reformat with MS-DOS (FAT), then see what happens when you copy back. If it doesn't work, chuck it. Since flash drives are so inexpensive now, get a few more of them in larger capacities.

Lastly, (above) some flashdrives have a physical write-lock (read-only) hardware switch. If your flash drive has such a switch, perhaps it is locked? OS X does not allow the usual file & directory permission on MS-DOS (FAT) disks, so you can't try to make it read/write.

There is also a 4GB limit on a single file in MS-DOS (FAT).

Answer 2

So, here's what I did after much face-palming and cursing of Apple and their absolute disregard for their users:

From the terminal:

1. Identify your USB by NAME and IDENTIFIER:

diskutil list

Output is:

/dev/disk3 (external, physical):  
#:                       TYPE NAME                    SIZE    IDENTIFIER  
0:               FDisk_partition_scheme              *2.0 GB     disk3
1:                 DOS_FAT_32 MIXTAPE                 2.0 GB   disk3s1

In this case, NAME=MIXTAPE and the IDENTIFIER=/dev/disk3s1

2. Now unmount the USB:

sudo diskutil unmount /dev/$IDENTIFIER

Example:

sudo diskutil unmount /dev/disk3s1

Output is:

Volume MIXTAPE on disk3s1 unmounted

3. Now create the Volume directory - this appears to be the key!

sudo mkdir /Volumes/$NAME

Example:

sudo mkdir /Volumes/MIXTAPE

No output.

4. Now mount the USB to the Volume:

sudo mount -w -t msdos /dev/disk3s1 /Volumes/$NAME

Example:

sudo mount -w -t msdos /dev/disk3s1 /Volumes/MIXTAPE

No output.

5. Validate that the USB is now writeable:

touch /Volumes/$NAME/tmp.txt

Example:

touch /Volumes/MIXTAPE/tmp.txt

6. You should now be able to see that you were able to create the tmp.txt file on your USB in the Finder app or by:

ls -al /Volumes/$NAME

Example:

ls -al /Volumes/MIXTAPE

Answer 3

This is a stab in the dark, but is your USB drive actually an SD card in a card reader? In this case, I would tend to think the hardware write lock switch is in the on position on the SD card.

Answer 4

Formatting is sure an a option, but what if that flash drive is not yours or you have info on it? IMO, the best possible way is to download something like Paragon NTFS for Mac. Had the same problem, downloaded, installed, no more problems, easy life.

Answer 5

If you want to use on Mac and Windows - Select the USB drive in the left hand pane. From the right hand side, choose Partition, change layout to 1, and format to ExFat, and see if that helps.

Answer 6

Going out on a limb here:

How many files are on the drive? FAT32 has a maximum number of files per volume. Assuming that the drive is formatted with 32KB clusters, only 268,173,300 files can reside on a FAT32 drive at once. Is the 1.09GB comprised of just under 268,173,300 small files?

Additionally, as IconDaemon mentioned briefly, FAT32 has a file size limit of 4GB with large file size support and 2GB without. How big is the file that you are copying to the drive?

FAT32 Wikipedia page

Answer 7

Have you tried rebooting with the drive in the USB slot?

I have a similar USB stick in FAT32 format that's 32gb total. I find the (almost) ONLY way I can mount it read-write in 10.12 Sierra or 10.13 High Sierra is to reboot the machine with the drive inserted. If I insert the drive after the machine has booted it will always mount read-only. There's no physical switch for disabling the read, but I suspect that somewhere Mac OS thinks that switch is enabled and somehow it doesn't check when it's booting. I did manage to remount it read-write as shown below, but rebooting (though frustrating) behaves better and feels safer than doing it the other way and getting messages about it being damaged.

The only other odd thing about the USB stick is that it has one USB A side and one Micro-USB OTG side. Only one side can be used at a time, both access the 32Gb FAT32 storage. I'm using the USB A side in the Mac. I wish they made these with A and C sides.

Well, I was able to mount it read-write with the following terminal fun, but after mounting it I got a dialog that said Repair failed, unable to repair this drive, it has been mounted read only for you to retrieve files from it, but you will not be able to write to it.

And then I could write to it. But it was kind of upsetting.

lamblincl5116:~ lamblin$ sudo df
Password:
Filesystem      512-blocks      Used Available Capacity iused               ifree %iused  Mounted on
/dev/disk1s1     976695384 602657408 368466424    63% 2088422 9223372036852687385    0%   /
devfs                  374       374         0   100%     648                   0  100%   /dev
/dev/disk1s4     976695384   4194344 368466424     2%       2 9223372036854775805    0%   /private/var/vm
map -hosts               0         0         0   100%       0                   0  100%   /net
map auto_home            0         0         0   100%       0                   0  100%   /home
/dev/disk2s1s10   62980000  15263712  47716288    25%       0                   0  100%   /Volumes/LAMBLIN
lamblincl5116:~ lamblin$ diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk1         500.1 GB   disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +500.1 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume Macintosh HD            308.6 GB   disk1s1
   2:                APFS Volume Preboot                 22.9 MB    disk1s2
   3:                APFS Volume Recovery                520.8 MB   disk1s3
   4:                APFS Volume VM                      2.1 GB     disk1s4

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *32.3 GB    disk2
   1:             Windows_FAT_32                         32.3 GB    disk2s1

lamblincl5116:~ lamblin$ sudo gpt show -l /dev/disk2
     start      size  index  contents
         0         1         MBR
         1        95         
        96  63012768      1  MBR part 12
lamblincl5116:~ lamblin$ diskutil unmount /dev/disk2s1
disk2s1 was already unmounted
lamblincl5116:~ lamblin$ sudo mount -w -t msdos /dev/disk2s1 /Volumes/LAMBLIN 
mount_msdos: /dev/disk2s1 on /Volumes/LAMBLIN: Resource busy
# Here I realized that the device name was unusual.
lamblincl5116:~ lamblin$ sudo umount /dev/disk2s1s10
umount(/Volumes/LAMBLIN): Resource busy -- try 'diskutil unmount'
lamblincl5116:~ lamblin$ diskutil umount /dev/disk2s1s10
Volume LAMBLIN on disk2s1s10 unmounted
lamblincl5116:~ lamblin$ sudo mount -w -t msdos /dev/disk2s1s10 /Volumes/LAMBLIN 
mount: realpath /Volumes/LAMBLIN: No such file or directory
lamblincl5116:~ lamblin$ mkdir /Volumes/LAMBLIN
mkdir: /Volumes/LAMBLIN: Permission denied
lamblincl5116:~ lamblin$ sudo mkdir /Volumes/LAMBLIN
lamblincl5116:~ lamblin$ sudo mount -w -t msdos /dev/disk2s1s10 /Volumes/LAMBLIN 
mount_msdos: /dev/disk2s1s10 on /Volumes/LAMBLIN: Permission denied
lamblincl5116:~ lamblin$ sudo mount -w -t msdos /dev/disk2s1 /Volumes/LAMBLIN 
# This is when I got the Unable to be repaired dialog box
# I wrote files to it in the finder, and pressed eject in the finder.
lamblincl5116:~ lamblin$ sudo umount /dev/disk2s1
umount: /dev/disk2s1: not currently mounted
lamblincl5116:~ lamblin$ ls /Volumes/
Macintosh HD Preboot 13   Preboot 19   Preboot 24   Preboot 3    Preboot 35   Preboot 40   Preboot 46   Preboot 7
Preboot      Preboot 14   Preboot 2    Preboot 25   Preboot 30   Preboot 36   Preboot 41   Preboot 47   Preboot 8
Preboot 1    Preboot 15   Preboot 20   Preboot 26   Preboot 31   Preboot 37   Preboot 42   Preboot 48   Preboot 9
Preboot 10   Preboot 16   Preboot 21   Preboot 27   Preboot 32   Preboot 38   Preboot 43   Preboot 49
Preboot 11   Preboot 17   Preboot 22   Preboot 28   Preboot 33   Preboot 39   Preboot 44   Preboot 5
Preboot 12   Preboot 18   Preboot 23   Preboot 29   Preboot 34   Preboot 4    Preboot 45   Preboot 6

Answer 8

I found that the trick was that the volume name cannot have blanks in it. No matter what I tried, when the volume name contained a blank, I could not make the volume read/write.

Answer 9

Sticky tape over the lock switch - sm sq, ~15 mm, diagonally, not over card contacts.