The Universal Disk Format (UDF) is a format specification of a filesystem for storing files on recordable media. It is an implementation of the ISO 13346 standard, and an extension of ISO 9660. It is mainly used on media with limited rewriting conditions such as:

Lately, however, it has also become popular with large and fully rewritable cross-platform media such as:

It is much used for DVD-Video discs, but is also supported by burner software such as DirectCD, InCD, Toast, and Write UDF and directly by new operating systems, such as Linux, with the 2.6 Kernel, and Mac OS X.

UDF is practically the successor of ISO 9660, supporting larger files, larger disk and more information about individual files and folders. It includes support for special file properties, such as Apple's File Types, Resource Forks, and other OS-specific data.

UDF has been released in multiple revisions:

Modern operating systems understand 1.02 and 2.01 Revisions of UDF (MacOS X (up to 10.3) only supports Revision 1.02)

UDF Revison 1.50 is used for so called "packet-written" writable CD-ROMs. Packet writing allows CDs to be partially written using variable or fixed length records in multiple Sessions. Variable length packets written CD-Rs and CD-RWs can usually be closed to ISO 9660 format just by writing a table of contents on the CD. A Fixed length packet formatted CD can not be closed to ISO 9660 format, but can be randomly written and overwritten. Fixed length formatting cuts CD capacity by about 20% relative to ISO 9660 or variable length formatting.

Older operating systems do not support UDF unless special drivers are loaded; for example Microsoft Windows before the late 1990s. On these systems, UDF-formatted media cannot be read without closing the CD to ISO 9660 format. Once closed, the CD cannot be edited any more.

In conjuction with packet writing, UDF Revision 1.50 allows a CD-RW to be used like a floppy disk.

A contradictory rumour says Mac OS X 10.3.2 mounts and reads (but does not write) UDF 1.50 on DVD-RAM discs erased via such commands as Linux's mkudffs --udfrev=0x0150 /dev/scd$n.

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