sd card image erase--potentially harmful to cards?

Hi,
I've been taught 2 apparently contradictory things re: erasing images v. reformatting cards. I was taught that: 1) it's fine to look at images on the back of the camera and erase anything you want as you go. I've also been taught that you NEVER EVER erase an image on a card--that you back up and load your images and then reformat the card to erase images. It was explained that doing it any other way could potentially corrupt the card or, at least, cause there to be problems with images. I'm inclined to "believe" --sort of intuitively-- that I should download the images and then reformat the card and should not erase images 1 at a time in camera. I'd like to know if I'm right and, if so, could someone please explain to me the WHY of it?
Thanks
Stephanie

I see you are on a Mac. . .

Reformat the card in the camera. . .

If you "erase" or delete them on the mac they are simply moved to a hidden folder and take up just as much space.

Since switching from a PC, I've gotten into the habit of reformatting the card immediately after inserting it into the camera.

not sure how deleting a file on a storage medium would cause corruption personally..

Under normal reading/writing/deleti​ng of any files should not cause corruption of your images. Any "you should format each time" or "you should not fill up the card to the max" are just preventive measures. Sometimes, cards just go bad no matter what you do.

Also, if all you do is delete files, especially one at a time, you leave a lot of other stuff on the card. Over time this will fill and fragment your card, increasing risks.

Format or erasel all makes no different in flash memory operation. Deleting one file/picture is alittle different, but they are not the reason you get corrupted files.

When you format or erase all on storage flash memory, you actually do NOT delete any files. The file system simple remove the "pointers" to your file, or simple put, the file system mark the space as empty. When you write new files, the file system then erase the neccessary location and write the new file. This is why you can recover your file even after you delete them or format the card.

Deleting one file involve a slightly more complicated operation, but basic ideal is the same. The file is actually NOT deleted until its space is needed.

Now, with low level format, which usually requires special software, you can actually erase everything single location on a memory card.

eel

GSansoucie wrote in post #6023579
If you "erase" or delete them on the mac they are simply moved to a hidden folder and take up just as much space.

i hate that!
especially with flash drives.
the "hidden folders" are put in the trash so as soon as you empty the trash ur stuff is gone.

I erase images from my cards all the time. If I don't like a shot or I trip the shutter accidentally, I'll delete the image from the card right then and there. Once I get the images uploaded to the computer I format the card in the camera. As of yet, I haven't had a card fail on me and I've been using the same cards since 2002.

Jon wrote in post #6025384
Also, if all you do is delete files, especially one at a time, you leave a lot of other stuff on the card. Over time this will fill and fragment your card, increasing risks.

Thanks Jon...I think this is what I was trying to say. If I'm correctly understanding what you are saying, it seems as if one way to think about it is that it is sort of analogous to deleting files on a computer. Sooner or later you end up needing to run a defrag program which will recover space and compress unusable bits of space. The big difference is that you can't run a defrag program on a SDHC card. Is that more or less correct? If it's on the right track, here's the follow up question: how does low level format fit into the overall picture? My personal process is that I do a day's shooting and then take that card and download it to my computer or personal data storage device and verify that all is there. Then I format the card and every few formats I choose to do a low level format. I have absolutely no understanding of why I do the low level format or whether the process I use is good or not.
My original question was a poor attempt to try to understand the relationship between erasing single images in camera and then either reformatting or doing a low level format. Do I in anyway defrag the SDHC card by doing the low level reformat? I still don't quite "get it."
Thanks for the help
Stephanie

Flash memory devices are somewhat less susceptible to fragmentation issues since they typically incorporate a "wear-leveling" algorithm so even if you're writing to sequential memory locations, you're not allways writing over the same part of the card. However, if you have a bunch of directories, especially, left over due to deleting photos one-by-one, you make things somewhat more difficult for the algorithm.

AFAIK, a "basic" format of a flash device wipes the file allocation table and root directory; a low level format of an SD-type card (not an optional choice for CF cards) also verifies the rest of the card. So either operation will leave you with a shiny fresh card as far as your camera's concerned. A low-level format, however, will let the camera or computer mark out newly-bad sectors so you don't try to write to them.

Jon's got it right with the algorithm.
Every day I deal with linear and ATA PCMCIA cards and CF cards.
When writing to these types of solid state media; the writing is not sequential sector by sector order.

There is a controller in the memory card that directs the data to random sectors to spread out the wear on the card. Defragging a solid state card renders this process useless.

thanks all...
Stephanie