Erasing magnetic media has been investigated in detail, with 3-pass, 7-pass and 35-pass methods developed.
However, a Solid State drive, and SD card or a thumb drive, represent flash media that do not readily rewrite every bit, when formatted.
A simple strategy to employ, would be to format the drive as a Truecrypt whole drive partition. However, upon completing the format, one need not use the encrypted area. Simply mount it and format it (a requirement,) and then unmount the drive and quick format it for NTFS or FAT32, using the operating system utility.