Can and should UDF be used as a hard drive format?

Solution 1:

Someone did some research into how to format a flash drive with udf so it can be used on as many operating systems as possible. Here are his findings (used to be there, now offline):

• Windows 7 have full support up to UDF v2.6, but the UDF block size must match the block size of the underlying device (which for USB-sticks and most disks is 512 bytes; "advanced format" disks are 4096 bytes). Apparently the disk must be partitioned.

• Linux 2.6.30 and up supports UDF fully at least up to version 2.5.

• Mac OS X 10.5 supports UDF fully up to UDF 2.01, but only when used on a full disk, so not partitioned.

As explained above, for USB harddisks, Windows requires the disk to be partitioned. On the other side, UDF only works in OS X when it is used on a full disk (unpartitioned). Rather surprisingly, there is a solution which works for both: having the disk partitioned and unpartitioned at the same time.

DOS partition tables are stored in bytes 446-510 of the master boot record. This master boot record is stored in the first sector on disk, sector 0. Typically, the first partition specified will start some kilobytes further. However, it seems possible to construct a partition table whose first partition starts at sector 0, so the result is a partition which contains the partition table itself. Partition editor programs seem to refuse to create such a table, but at least recent Linux and Windows kernels don’t seem to bother.

The nice thing is that UDF does not (I suppose deliberately) use the first few kilobytes of the partition or disk it is placed on, so this place can really be used to store a legacy partition table, referring to a partition that spans the whole disk. Some testing shows that this really works on Linux, Windows and Mac OS X:

• Mounts automatically read-write in Linux 2.6.30+, Mac OS X 10.5+, Windows Vista+
• Can be used read-only in Windows XP, and be used after a command line mount in Linux 2.6.0+
• Supports large files, UNIX permissions, Unicode filenames, symlinks, hardlinks, etc.

Script to format the disk properly: Perl script or Bash script

Solution 2:

No.

We're in 2015 at the time of this reply. I am using OSX Yosemite, Ubuntu 14.10, and the Windows 10 technical preview for enterprises on a Mactel machine (Macmini 7,1).

I tried both UDF and exFat. I use Ubuntu for development and do need Unix-style permissions.

All former guides do not apply anymore: UDF drivers have evolved and all operating systems will accept a UDF partition, with more problems and instabilities than I can name.

• UDF drive formatted on Mac OS: can't be mounted on Windows 10.
• UDF drive formatted on Linux: can't be mounted on Windows 10.
• UDF drive formatted on Windows 10: mounts read/write on Linux, read-only on OSX.

However, Windows doesn't allow you to specify a block size when formatting a UDF volume, and as a result, your logical block size might differ from the physical block size for the partition.

I am unclear whether this has to do with the difficulties I had mounting it read/write on OSX, but after deleting a certain number of files using Linux, I was never able to mount the drive again on OSX.

The system goes into kernel panic and crashes disgracefully.

This, and a variety of answers on the subject, indicate inconsistent support for this format at this point.

It would seem there are ways I can use a NTFS volume to achieve a balance between the features of a modern file system, Unix-style permissions - I might be able to set them - and read/write mount on all operating systems.