Editing Universal Disk Format (section)

=== Spared (RW) build ===
Rewriteable media such as [[DVD-RW]] and [[CD-RW]] have fewer limitations than DVD-R and CD-R media. Sectors can be rewritten at random (though in packets at a time). These media can be erased entirely at any time, making the disc blank again, ready for writing a new UDF or other file system (e.g., ISO 9660 or [[CD-DA|CD Audio]]) to it. However, sectors of ''-RW'' media may "wear out" after a while, meaning that their data becomes unreliable, through having been rewritten too often (typically after a few hundred rewrites, with CD-RW).

The plain and VAT builds of the UDF format can be used on rewriteable media, with some limitations. If the plain build is used on a -RW media, file-system level modification of the data must not be allowed, as this would quickly wear out often-used sectors on the disc (such as those for directory and block allocation data), which would then go unnoticed and lead to data loss. To allow modification of files on the disc, rewriteable discs can be used like -R media using the ''VAT'' build. This ensures that all blocks get written only once (successively), ensuring that there are no blocks that get rewritten more often than others. This way, a RW disc can be erased and reused many times before it should become unreliable. However, it will eventually become unreliable with no easy way of detecting it. When using the ''VAT'' build, CD-RW/DVD-RW media effectively appears as CD-R or DVD+/−R media to the computer. However, the media may be erased again at any time.

The ''spared'' build was added in revision 1.5 to address the particularities of rewriteable media. This build adds an extra ''Sparing Table'' in order to manage the defects that will eventually occur on parts of the disc that have been rewritten too many times. This table keeps track of worn-out sectors and remaps them to working ones. UDF defect management does not apply to systems that already implement another form of defect management, such as [[Mount Rainier (packet writing)|Mount Rainier]] (MRW) for optical discs, or a disk controller for a hard drive.

The tools and drives that do not fully support revision 1.5 of UDF will ignore the sparing table, which would lead them to read the outdated worn-out sectors, leading to retrieval of corrupted data.

An [[overhead (computing)|overhead]] that is spread over the entire disc reserves a portion of the data storage space, limiting the usable capacity of a [[CD-RW]] with e.g. 650 MB of original capacity to around 500 MB.<ref name=PC-Thompson>{{cite book| url = https://books.google.com/books?id=kG8LcWfruOAC&pg=PT242 |title = ''"PC Hardware in a Nutshell: A Desktop Quick Reference"'' by Robert Bruce Thompson, Barbara Fritchman Thompson (2003) −– Chapter 11 (starting page 340): CD writers| isbn = 9780596552343| last1 = Thompson| first1 = Robert Bruce| last2 = Thompson| first2 = Barbara Fritchman| date = 24 July 2003| publisher="O'Reilly Media, Inc." }}</ref>