SD card 'empty' on camera and MacBookAir but on photo center reader it's almost full

I recently downloaded from my Email some pics someone sent me from their iPhone. I have a MacBookAir. I have an older Canon PowerShot A530 camera I use as a casual photographer.

I put the SanDIsk 2GB card (older card) in my computer and went to ‘Finder’ to move the pics to the card-  they appeared to have gone there.- but not into the DCIM/ 100 Canon folder. I got to the RiteAid photo center I usuallly go to make prints from my SD card and when it opened the machine told me not all the files could be opened. Only 2 of the 8 pics I downloaded showed, along with 4 existing pics on the card (that I’d taken with the camera) BUT-then all these old pictures showed up on the screen- pics I’d taken a long time ago and erased off of the card. (I can’t remember if I erased them from the camera or from the ‘Finder’ on my MAC). They are taking up space on the SD card- now I know why I can only take so many pics before a message: “Memory card is full” comes up on my camera when there aren’t many pictures showing on the card while in the camera.

How do I get them to show up so I can delete them off of the card?

The next question I have is- I fully emptied the SD card on my camera and then tried to drag the downloaded pics to the SD card again after inserting it into my Mac port. The picture showed in the ‘Finder’ under ‘Remote Canon’ (not in a folder) but when I put the card into the camera it read ‘no images’.  Why am I able to drag the pics into the Canon link in the ‘Finder’ but they aren’t showing in the camera?

Thanks to everyone who replies…

It is a common error and you might need to use a good photo recovery software. Sandisk has some photo recovery solution as far as I know. And if none of that works, Try Stellar Phoenix Photo Recovery, this one worked for me.