Everyone (I imagine) has seen what a microwave can do to a CD/DVD/Bluray. It's not "completely" trashed, but for most investigative endeavors, microwaved DVDs are just trash.

Flash drives, however, can survive microwave radiation for a considerably long time. Don't microwave them. Encrypt them with very very good encryption that does NOT have an FBI back door built in. (See Veracrypt.) But, if you do need a clean flash drive, fill it with a movie or something. Then run Eraser (or some military wipe software).

If anything sensitive has ever left your computer and hit the internet in any way, your goose is cooked.