This article is from the Computer Security Evaluation FAQ, by Trusted Product Evaluation Program TPEP@dockmaster.ncsc.mil.
In summary, no; in general, overwriting may be sufficient to have
media released for other use, but it must retain its original
classification.
You should contact your security officer or contracts manager for
official guidance. Often, your contract will determine how to
declassify disks. This is usually indirect, by referencing a
DOD-STD or other document. Be prepared to submit the disk drive
(or at least the little metal thingy with the iron oxide) for total
destruction.
If you need to retrieve unclassified data that reside on a
classified disk, there are often detailed procedures to accomplish
this.
 
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