Universal Disk Format
The Universal Disk Format (UDF) 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 90's. On these systems, UDF formatted media can not be read without closing the CD to ISO 9660 format. Once closed, the CD can not be edited any more.
For the basic user UDF allows a CD-RW to be used like a floppy disk.
More generally, UDF is a format specification of a file system for storing files on recordable media, mainly media with limited rewriting conditions, such as:
Lately 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 and Write UDF and directly by new operating systems, such as Linux 2.6.
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 is fully compliant with the DVD-Video disks.
Modern operating systems understand 1.02 and 2.01 revisions of UDF (Mac OS X (up to 10.3) only support 1.02)
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 `mkudffs --udfrev=0x0150 /dev/scd$n`.
