(This is a follow-up to "What is the partition id / filesystem type for UDF?")

I know two ways to format a hard drive as UDF:

  • Windows Vista or later: "format x: /fs:UDF" (don't use /q ! )
  • Linux: "mkudffs --media-type=hd --blocksize=512 /dev/sdx"

The problem is that the 'other' OS does not recognize the disk as formatted at all: it simply refuses to mount it, no matter what commands I try.

How can I format a hard drive as UDF so that both Windows and Linux will be able to use it?

EDIT: updated the commands, now the result should work in either OS.

  • UDF was designed for optical media, perhaps that's the problem? – Chopper3 Aug 17 '09 at 16:32
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    The command you give works great on Linux. I formatted on Karmic, and can read and write under both Windows 7 and Karmic. – Matt Joiner Dec 29 '09 at 13:44
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    Do not forget to zero the MBR first (dd if=/dev/zero of=/dev/sdx bs=512 count=1), to avoid confusion with any leftover partition table (since UDF does not use the first sector). – CesarB Jan 2 '10 at 2:59
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    Related question: superuser.com/questions/39942/using-udf-on-a-usb-flash-drive – CesarB Jan 2 '10 at 3:03
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    Thanks Skolima, this is great. Bye bye FAT32. I only need to be careful to stick to UDF version 2.01 and to 512 block size and I have the perfect hard disk partition shared across Linux and Windows. – MarcH Dec 29 '11 at 23:46
up vote 16 down vote accepted

It turns out that using the /q switch on Windows was the culprit: it enables 'quick format', i.e. the formatting process continues in background with every write made to the disk. Once it finishes, the drive is handled by Linux just fine.

  • On the other hand, formatting with Linux' mkudffs seems to sucessfully complete in seconds. – MarcH Dec 29 '11 at 23:41
  • I actually had to do a quick format in Win7 after creating the file system with mkudffs in Linux. Without that, Windows just kept wanting to format the drive. It ended up creating a 5MB empty space at the end of the drive, for whatever reason. But at least it works in both OS now. – DanMan Nov 3 '12 at 23:41
  • In reply to myself: those 5MB might be the meta data block in recent UDF versions. – DanMan Sep 8 '13 at 11:44
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    Totally wrong! Quick formatting is NOT "continuing format in background". It is just initialization of the file system without formatting tracks. – Anixx Sep 19 '16 at 12:43

When formatting UDF, one thing that can't be underscored enough is how important it is to use the proper block size when formatting UDF. As explained here, using a configured block size that doesn't match the physical sector size likely means that some operating systems will not be able to detect the drive as valid UDF.

Inspired by the above-linked answer (and my other research/testing), I wrote a script to automate the process of formatting in UDF--using the properly detected sector size. See format-udf on GitHub. Notable features:

  • Formats a block drive (hard drive or Flash drive) in Universal Disk Format (UDF)
    • UDF revision 2.01 used for maximal compatibility
    • First 4096 sectors are zeroed out to erase any existing MBR (necessary for proper UDF detection)
  • Resulting file system can be read/written across multiple operating system families (Windows, OS X, and Linux)
  • Runs on any OS having a Bash environment

Because of the last point, this script I wrote cannot be used on Windows. However, the script will run on OS X and Linux. After doing so, Windows should be able to magically detect the newly formatted UDF drive.

To directly answer the questions posted, format-udf will:

  • choose the appropriate tool for formatting based on operating system and environment (mkudffs on Linux)
  • automatically detect and populate all parameters necessary for formatting (including blocksize)
  • maximize OS compatibility (see GitHub page for compatibility chart)
  • yield the maximum feature set (and minimal limitations) that the asker is looking for

How did you reconcile your discovery in the previous question (that the UDF filesystem should be created on the whole disk, not a partition) with that Windows command ("format x: /fs:UDF")? In my attempts, Windows only gives drive letters to partitions.

As far as formatting for compatibility, I think the key is in the block size. Since most hard drives and USB flash sticks have a block size of 512 bytes, I've had the most compatibility when I create the FS that block size. I think format.com is using that block size, and mkudffs has a command switch for changing the block size. I could only get OS X and Windows to mount the filesystem when I used 512 byte blocks. Older versions of Linux assumed a block size of 2048, but you can always mount with "-o bs=512".

The whole disk vs partition issue still causes compatibility problems. Windows won't mount when I format the whole disk, and OS X doesn't look beyond the partition's type number when determining its filesystem, forcing me to mount it manually. Linux didn't care, as long as I gave it the appropriate device name (sda vs sda1).

In summary, the most compatible setup I've found is a singe partition of type 06(FAT16), formatted with UDF at block size 512. Works automatically on Windows, and a small bit of manual intervention on Linux and OS X.

  • 3
    I've tried another experiment. This time I used a USB flash stick instead of a hard drive. Formatted with "mkudffs --media-type=hd --blocksize=512 /dev/sda", works without hassle on Linux, OS X, and Vista. Too bad Windows seems to allow partitionless formats on USB sticks but not hard drives. – Simon Oct 9 '09 at 18:22
  • You definitely want to keep the same block size to avoid prematurely wearing the flash memory, see: lwn.net/Articles/428584 – MarcH Dec 29 '11 at 1:22

protected by Mark Henderson Jul 12 '12 at 20:30

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