Hi and welcome to the forum:
This issue is not so much to do with cameras, but with the nature of file management systems, and how different systems can cause issues.
Cameras use a FAT 32 file system, that uses a 32 bit file structure to identify each file. The drives that store them - be they cards or hard discs, use a file table of contents called a File Allocation Table (hence FAT). This FAT table stores the file name along with the addresses on the drive within which the data is located. On a clean drives the files are stored and laid continuously which keeps the FAT table pretty simple.
When a file is 'deleted' by you, the actual data is not deleted - what happens is that the first element in the file identifier is replaced with a 0 - essentially indicating that the space used by the file is available to be over-written. Now, when the card is removed from the camera and used on a system with a different filing system - like NTSF of variants of the Apple FMS, then that operating system's file manager may actually read the FAT and, if the addresses have not been over-written, consider it still active, thus you will see that the files are still there.
Simply deleting files on your camera's cards is not good disc management hygiene. Because when a file is identified as 'deleted' another file will be written to that space, but the issue is that if the block of data for the new file does not exactly match the size of the new file (which is extremely likely), then that file will be split between the current deleted file space and another location, which may itself be overwriting an existing deleted file. That may then leave a smaller amount of spare space that will itself be used to create a fragmented file over multiple addresses. As this goes on, the number of addresses required to identify the multiple locations of a single file become more and more complicated. This can lead to corruptions of the FAT table and loss of files, or even the unrecoverable corruption of the whole card (or thumb drive in other situations).
For that reason, the recommended protocol is to start with a clean card. After shooting, copy the files across to a computer hard disc and, after confirming the files are there, remove the card, return it to the camera and format the card clean in the camera using its file manager to start the new shoot. This assures that the files are written contiguously, with the greatest speed and the most efficient use of the FAT because the format process creates a new FAT table.
Once the files are copied across to the hard disc, that is when you delete files. this is far less likely to cause and is with computer hard discs because they are managed by a far more sophisticated (and larger) operating system and file manager that will not fit on a camera. Such systems run disc optimization algorithms in the background to clean up the FAT table and thus keep the system clean and efficient. In earlier systems you had to do this through a program called a disc defragmentation tool, but that is not built into the system and fragmentation is far easier to manage with solid state drives than the traditional ones with platters that caused a phenomenon called disc latency.
cheers, TREVOR
The mark of good photographer is less what they hold in their hand, it's more what they hold in their head;
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
