Wear leveling can also defeat data erasure, by relocating blocks between the time when they are originally written and the time when they are overwritten. For this reason, some security protocols tailored to operating systems or other software featuring automatic wear leveling recommend conducting a free-space wipe of a given drive and then copying many small, easily identifiable “junk” files or files containing other nonsensitive data to fill as much of that drive as possible, leaving only the amount of free space necessary for satisfactory operation of system hardware and software. As storage and system demands grow, the “junk data” files can be deleted as necessary to free up space; even if the deletion of “junk data” files is not secure, their initial nonsensitivity reduces to near zero the consequences of recovery of data remanent from them. Optical media As optical media are not magnetic, they are not erased by conventional degaussing. Write-once optical media (CD-R, DVD-R, etc.) also cannot be purged by overwriting. Rewritable optical media, such as CD-RW and DVD-RW, may be receptive to overwriting. Methods for successfully sanitizing optical discs include delaminating or abrading the metallic data layer, shredding, incinerating, destructive electrical arcing (as by exposure to microwave energy), and submersion in a polycarbonate solvent (e.g., acetone). Data on solid-state drives Research from the Center for Magnetic Recording and Research, University of California, San Diego has uncovered problems inherent in erasing data stored on solid-state drives (SSDs). Researchers discovered three problems with file storage on SSDs: Solid-state drives, which are flash-based, differ from hard-disk drives in two ways: first, in the way data is stored; and second, in the way the algorithms are used to manage and access that data. These differences can be exploited to recover previously erased data. SSDs maintain a layer of indirection between the logical addresses used by computer systems to access data and the internal addresses that identify physical storage. This layer of indirection hides idiosyncratic media interfaces and enhances SSD performance, reliability, and lifespan (see wear leveling), but it can also produce copies of the data that are invisible to the user and that a sophisticated attacker could recover. For sanitizing entire disks, sanitize commands built into the SSD hardware have been found to be effective when implemented correctly, and software-only techniques for sanitizing entire disks have been found to work most, but not all, of the time. In testing, none