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UDF (Universal Disk Format)
The Windows UDF filesystem format implementation is OSTA (Optical Storage Technology) UDF-compliant, which supersedes the earlier filesystem CDFS (or CD-ROM filesystem) legacy format. UDF is a subset of the ISO-13346 format with extensions for formats such as optical media. Although the UDF filesystem was designed with rewritable media in mind, the Windows UDF driver (\System32\|Drivers\UDfs.sys) provides read-only support. Moreover, Windows does not implement support for other UDF features, including named streams, access control lists, or extension attributes. Vendors such as Ahead’s Nero Premium Reloaded and Roxio’s Easy Media Creator Suite, for CDs and DVDs usually use a technology called packet writing, and a format optimistically called UDF. The idea is that a packet-written UDF CD or DVD written on one computer is accessible and available on another. Unfortunately, packet-writing and UDF are not universal because different vendors use different versions of packet-writing; even with the proposed UDF standard. Consequently, drag and drop recording is convenient but it is not universal. To limit incompatibly and in order for the contents to be accessible on a packet written CD-R on CD-ROM drives the session must be closed. For packet written CD-RWs it is not necessary to close the session. The Purple Book Standard of 2000 and the current version of 2001, double-density CD-ROMs, e.g., DDCD, DDCD-R, DDCD-RW, media and drives are defined using CIRC7 over CIRC. Note: Some Book Standards, most in fact, use the features of each others Book Standard in one form or another in order to extend the capabilities the data stored on optical disc. The Orange Book Standard introduced multi-session recording, and packet writing which uses the relatively recent UDF filesystem with many advantages over the ISO 9660 CD filesystem, permitting the disc to act like floppy diskettes but more importantly treats recordable optical disc as if they were a standard magnetic disk. Modern CD-ROM drives are able to view the contents of an optical disc using UDF if they conform to the "MultiRead Specification" and the operating system must have installed a UDF driver which comes with authoring software such as Nero. The UDF Volume Reader driver can also be obtained freely from Roxio. If backing up and recoverability is the main concern use the "ISO 9660 standard. See: Disk & Disk.

Unallocated Space
Available disk space that is not allocated to any partition, logical drive, or volume. On basic disks, unallocated space outside partitions can be used to create primary partitions or extended partitions. Free space inside an extended partition can be used to create logical dos drives. On dynamic disks, unallocated space can be used to create dynamic volumes. See: Basic Disk, Dynamic Disk, Extended Partition, Logical Drive, Partition, Primary Partition, and Volume

Unformatted Capacity
The total number of bytes of data that can fit on a disk. The formatted capacity is lower because space is lost defining the boundaries between sectors. However, as most media is preformatted today, this confusing issue is fading away.

Universal Naming Convention (UNC)
A convention for naming files and other resources beginning with two backslashes (\\), indicating that the resource exists on a network computer. Universal Naming Convention (UNC) names conform to the \\servername\sharename syntax, where servername is the server’s name and sharename is the name of the shared resource. The UNC name of a directory or file can also include the directory path after the sharename, by using the following syntax: \\servername\sharename\directory\filename.See: Filename.

Update
To modify information already contained in a file or program with current information, i.e., an update may be a maintenance fix. See: Upgrade.

Upgrade
When referring to software, to upgrade existing program files, folders, and registry entries to a more current version. Upgrading, unlike performing a new installation, leaves existing settings and files in place. See: Update.