Can't erase two old videos

The IXUS 50 works with MMC and SD cards. Your new 4GB card is actually an SDHC card. While it fits into the slot, it is electronically incompatible and will not work. You need to use SD cards, 2GB and smaller.

Make sure that the card is 2GB or smaller, the camera only supports SD not SDHC so it won't recognize a card larger then 2GB. If it's 2GB or smaller format it within the camera and then you should be good to go.

If you've already copied your photo's to a computer, or had them printed, or otherwise have backed up your photo's, look in the manual for your camera for a "format media card" option, try that. This will erase everything on the external card and give you plenty of room.

It could be, that what you are seeing, is the INTERNAL memory is being taken up with your pictures. Connect your camera to your computer with a USB cable and you will probably find them.

This Canon uses an SD memory card (previous Canon used CF cards).
The SD memory card has a "slide switch" that locks the card.
Look at the SD memory card and move the switch back and forth, then switch it all the way to the UNLOCK position. Put back in the camera and format the card, then use it.

Since this is a 10Megapixel camera, use at least a 2GB SD memory card. A 2GB SD card will hold 700 pictures, or so, in the best setting.

Hope this helps!

I never herad of a camera function as a catagorical filling cabnet it puts info into its own default folder and increments the picture numbers,
If I am wrong show me the site that has your info?

Different generations of hardware may be at the root of this. Pages below may be of help.

How to Transfer Pictures from Camera to Computer Without Software

https://support.usa.canon.com/kb/index?page=content&id=ART138112

https://www.dpreview.com/forums/thread/3932763

Computer cannot detect SD Memory card on USB Solved Closed

try formating.
but u may lose the data
also see that u format through sd utility available on sd.com

if still a issue than get free replacement by kingston as it has 5 yrs warranty

Dear,
All just try this trick
Connect your phone to PC and try to acces the card on PC. Now format the card but, take the backup of data before formatting the card.
Regards,
Rakesh Kumar

I went onto the BestBuys site and found a link for compatible memory cards - The SanDisk SDHC isn't compatible with the Canon Poswershot SD750!! Only a 2GB card will work. The Lexar SD and the Kingston Technology 2GB will.