My Nikon D70S digital SLR camera recently has issues with memory cards. Even for cards that worked perfect in the past and that I re-formatted prior to the current use, it sometimes stops identifying the card (after I already took many pictures with no problem), and I have to eject and re-insert the card in order for it to work properly. Please advise.

Plz have closer look at your memory card. It has a lock button. Just unlock it & start shooting like a Hollywood Hero.
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Try formatting the cards in a PC before you put into the camera. Do the long format, not the quick. It will take some time, but give it a shot. If that doesn't work, buy new memory cards. Cards don't last forever, and it's not worth it to take pictures on a marginal card and then have them not work.

I found this answer on the Nikon web site.

3) Camera displays “CHA” in top LCD. The camera has detected an invalid partition type or is not working properly. Reformatting the memory card may allow it to be used. If an approved memory card causes the camera to display the "CHA" indicator contact the memory cards manufacturer for assistance.

Please note that the default disk format used in Windows XP is FAT32 and if a card is formatted with this setting on an XP system, a D1 series camera will report "CHA". Reformat the card as "FAT" to use in the D1 series camera.

Firmware problem. Contact Nikon.

You would have to get an SDHC to CF adapter which could be pricey and they are hard to come by.

try buying a different memory card... or just buy a new memory card and format that one ....

MEMORY CARD

Difficult to say without seeing whats going off, your best bet is to stop using the card and get a memory card reader its quicker and better, and all your pictures may just be there and formatting the card may solve the problems..

if all is still not all right (prior to formatting the card) the card may have a fault or be corrupted (assuming the camera is ok)

There are many programs that can help you recover faulty formatted or corrupt memory cards. Do a search on google and try a few as some are better than others and some work better in certain situations than others.
searches to try
memory card recovery
memory card repair
Corrupted Memory Card

I wish you the very best of luck..
If I can be of further help drop me a line

Always format the memory card on the camera, using the menu or shortcut key combination, not on the computer. Formatting from Windows is not the same thing.
Have you done a format from the camera itself?
Is the memory card write-protected accidentally?

Although formatting the card should take care of that, I have used cards to transfer data from one computer to another, then the folder on the card is invisible to the camera, but still taking up space. Empty the card via the card reader by dragging files to the trash, then emptying the trash.