whats the difference

Then why does my card show more space when I format, rather that just erase all?

Good question. This is one of those places where the details aren't well documented. My best guess is that it has to do with the way they try to keep writes balanced across the card, but that's only a guess.

monkey_wrench wrote:
but i have been formatting my card everytime i need to erase images because it is alot quicker could that reduce the amount of images the card can store because its gone from 120ish to 77 in Large Super fine

Have you changed the ISO your shooting at ? I have a 20D (and a Pro 1 on its way) and there is a dramatic decrease in the number of images my 2Gb cards can store dependant upon the ISO setting. I read somewhere it's somthing to do with the additional noise in the image.

You'll also find that it gets more acurate about available capacity when your filling it up, as at the start it has to estimate the average size of what each image will be. And depending upon the complexity of the image taken, the amount of compression (JPG) will vary.

If your card has less MB after formatting, than it was most likely a good thing to do.

Among other things, format tests the disk and maps out bad sectors that are found to ensure no data will be lost in the future.
If your card has defective sectors, you might lose images that are stored in them.

Best regards,
Andy

At least on the G6, formatting does NOT test the card. Testing would take a long time, significantly longer than simply erasing all images. Also, depending on the technology, there is very little risk of losing an image once it's been successfully stored.

The whole "testing for defective sectors" thing was true for hard disks 20 years ago. It's not done by user utilities any more. For both flash and disks, it's done at the factory (and it takes a LOOONG time).

"Formatting" a card involves writing a few special values to a few special locations, and nothing more. That's why it's so fast.

Good point.

That's why I always format the CF card first on the PC and then on the camera. Should have mentioned it

Still, I heard that sectors on CF cards can go bad and are mapped out. Maybe just superstition on my part.
Also should have mentioned that I use some 1 gig microdrives (which have disks inside).

Best regards,
Andy

Andythaler wrote:
Good point.

That's why I always format the CF card first on the PC and then on the camera. Should have mentioned it

Still, I heard that sectors on CF cards can go bad and are mapped out. Maybe just superstition on my part.
Also should have mentioned that I use some 1 gig microdrives (which have disks inside).

Best regards,
Andy

Unless formatting on the PC takes a really long time, there is no testing going on. It's simply not possible to test without taking time.

It is certainly true that memory cells can go bad on CF cards. In fact, CF cells (and other NAND-technology digicam flash, such as SD/XD etc.) are so inherently unreliable that they include on-chip error correction to make up for the problem. For this reason, though, there are higher-level mechanisms that automatically map out bad areas of the card. (The same is true of hard drives.) Formatting will not recover or rejuvenate bad sections on either CF or disk. As on the camera, "formatting" on your PC with the standard Microsoft (or Apple) software doesn't run tests. It just puts some standard information into a standard place.

Regarding microdrives, they fall squarely into the category of "modern disk drives." Their low-level formatting and testing are done at the factory. There is no way for you to redo the low-level format without equipment that would cost at least hundreds of thousands, and probably millions, of dollars.

Finally, I can't resist noting that a lot of this confusion is Microsoft's fault. Way back in the MS-DOS days, they ignored industry convention and created a command called "format" which did nothing of the sort. They still refer to "formatting" a disk when what they are really doing is "setting up" or "making a filesystem".