flash loses charge from the cells. hard drives are mechanical, but they loose the magnetic charge at a significantly slower rate (or I should say, flash memory loses it faster).
but in both cases the files are better to be scrubbed regularly. zfs can do this with little effort, but creating once and then rechecking a checksum file could also have the same effect. gives the drive a chance to recognize and renew weak charges. I don't think flash drives and sd cards have such built-in functionality, so maybe have 2 partitions and copy the whole contents in a direction, overwriting data, next time the other direction. but always check the checksums before any such copy.