Unfortunately, from what I could glean from data recovery experts such as strategic technical alliance manager Chris Bross of DriveSavers and SMB partner manager Leon Feldman of ACE Data Recovery, some disk vendors put off block erasures for long periods of time or until they're forced to resort to them. Sad but true: You can't rely on housekeeping to remove data. Even sadder, there seem to be no utilities that will force the garbage collection. That seemingly simple solution has so far been ignored.
USB flash drives don't support standard ATA secure-erase or TRIM — so unless you're using a secure, encrypted type, you'll need to contact the vendor for an erase utility.
You could overwrite the entire drive or just free space with files. This will work to a point, but — especially on SSDs — some blocks used in over-provisioning and marked as bad can't be copied over. They may retain data you want to erase.
In the end, the only sure way to remove all unwanted sensitive data from the free space on an SSD or USB flash drive while retaining the data you still want is to back it up (use imaging if an operating system is involved), secure-erase the drive, and then restore the desired data. Sigh.
And when that's not enough...
All the methods and programs I've described will work great for the average user. That said, forensic data recovery technology has come a long way. Normal affordable methods won't counteract anything I've discussed. But if you have a formula for cold fusion, or a trade secret that will topple the global economy overnight...go for the degausser, the hammer, and then the blast furnace. You can't be too sure.
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