Unlike internal solid state drives, recovering deleted files from USB flash drives isn't difficult. Formatting USB flash drives will help prevent recovery, but only if you use the normal format.
Our friends at the How-To Geek explain that data recovery of internal SSDs is nearly impossible, because of how the TRIM command works. TRIM optimises SSD performance by clearing deleted data from the SSD immediately. Unfortunately, this only works on internal drives, which means flash drives (and external SSDs) are susceptible to data recovery techniques.
To prevent the recovery of deleted files from an external SSD or USB flash drive, you can format them, but make sure to uncheck the Quick Format box in Windows. Only the regular format command will erase data on the disk and make it hard for someone to use something like Recuva to undelete your files.