Hugh,
The files that you think you deleted from your portable drive using the Windows “delete” command or the Mac “move to trash” command are still there; you just can’t see them. Learn how to securely delete files from the .trashes and hidden files from thumb drives by doing an Internet search on the topic of .trashes file and how to securely delete old files from them. This is a must do for anyone using a USB thumb or SD card. If you have a Windows computer and can find the option to “show hidden files” you should do that. Look into using Apple’s FileVault.
The mini-SD card is small enough that it can easily be concealed underneath a round band-aid stuck to your skin.
RT
Highly recommend the use of ‘BleachBit’ on a regular basis to clean out the things remaining in the corners of your system. Installing it also gives you a right-click choice on files to ‘Shred’ them. Remember if you really want to get rid of something a good choice is to encrypt the file and then shred it, especially if you make up a long random encryption key of keyboard mashing….