8.2 Magneto-Optical Discs  The first (2004) edition of TC-04 described, as a possible target format, Magneto-Optical Discs. By the time of publishing it had reached a capacity of 9.1 GB. This development marked the end for this technology, and the format now has to be considered as endangered. The consequence of this is that the media and carriers will in time become difficult, if not impossible, to obtain. All content on M-O disc should be marked for migration to an appropriate storage system.  There has however been developed a new format that uses the same standardised 5.25 inch caddies as the MO disks, called UDO, (Ultra Density Disc). These discs use a phase change technology similar to CD-RW, and differ only from these in that they come in MO style caddies that protect the discs. Some hardware systems allow for use of both MO and UDO technologies in the same robot. A blue laser (405 nm) is used with a double-sided disc. The first UDO was presented in fall 2003 with 30 GB storage capacity. UDO disks are currently available with a 60 GB capacity, with a roadmap promising 120 GB in the next year, and a speculation on 500 GB as the ultimate target  Testing and Arrhenius extrapolation have estimated a life expectancy of up to 50 years. As discussed above in relation to other media, such testing should be considered cautiously. It is also much more likely that format obsolescence will be the ultimate limit of long term viability. Though UDO has some adherents, the technology has not penetrated the market to any extent and is consequently a risk for long term archival storage.  Though technological developments provide the pathway to long term preservation of our audio content, it behoves the curator, archivist and technician responsible for an archival collection to take a conservative and careful approach in the adoption of any new technology.