I have a Win10 System and a 250 GB Micro SD Card.

I have a Folder that is 243 GB and I was going to Copy it to My 256 GB Micro SD Card.

Every Time I Try it a Screen opens and Tells Me You Need 34.2 GB Of Free S[ace.

The Micro SD Card has nothing on it I Formatted it.

And Yes I can Copy other Files to it.

What am I not Understanding?

 

It seems to be saying you can't get that close to the limit

 

What am I not Understanding?

 

Ok I just Formatted the Micro SD Card again.

After the Format I Open the Explorer Window and Find My Micro SD Card.

Under the Micro SD Card it Says.
249 GB Free Out Of 256 GB.

What is this I have nothing on the Micro SD Card yet?

 

Every storage medium has a file structure and a directory. All of this require overhead and space for this must be reserved. Hence you will not have access to 100% of the raw storage space.

 

To amplify @MrChips comment: Physical media needs various reserved areas to be useful to an operating system. The device needs a file system in place so that when you want to store files on it, it can be organized in such a way the information can be found after it is written.

There must be some sort of index that points to the location of the various files on the device. As the device is used, files are written and erased (actual their space is marled as free) and so the free space is not necessarily contiguous. This means that any particular file you write to it may be located in many places on the physical device, but somehow the OS needs ot be able to know it is one file.

There will also need to be a reserved areas to compensate for bad blocks. That is, there needs to be some room not avaialble directly to the OS to deal with areas on the device that do not function properly. When a bad block is found, it is marked in the index with the alternate location.

The larger the device, the more space is needed for the index, of course.

So if you were ot make a bit-wise copy of something to the drive, that is, writing out the ones and zeroes directly with no file system data, it would hold the 256GB on the label. But in the course of ordinary use, a file system is part of what you are storing on the drive, to make it useful to you. That takes up space.

 

Also, the formatted Byte size on the drive matters because every file wastes a little bit of memory.

And lots of small files waste more space than fewer large files.

 

Another common misconception arises from the application of standard metric prefixes to non-decimal quantities. For example, It was popular in the early days of computers for 1 kilobyte to be equal to 1024 bytes. The relative error in this approximation is 24/1000 or 2.4%. Probably not a big deal. Using the same rule, 1 GB is actually equal to 1,073,741,824 which is (2^30). This is a relative error of 7.37%. You can see the pattern. As the numbers get larger so does the absolute error.

It matters if you understand a gigabyte to be 1e9 bytes or 2^{30} bytes.

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What is this I have nothing on the Micro SD Card yet?

The "problem" is caused by the difference between a kilobyte and a kibibyte. A kilobyte (KB) is 1000 bytes while a kibibyte (KiB) is 1024 bytes.

To add to the confusion, a KB on Windows is 1024 bytes (instead of 1000) as evidenced by the capacity reported for my 3TB and 2TB drives:

3,000,000,000,000 bytes = 3TB = 2.73TiB. Subtract some overhead for the filesystem and 2.72TiB seems reasonable. The problem is that Windows ignores the distinction between TB and TiB and uses the wrong label.

This is happening all around you and you only care when the imprecision causes a problem.

I have 16GB microSD cards from different manufacturers and the actual capacity isn't necessarily the same. It only causes a problem when you need those last few bytes. In my case, I was copying images for Raspberry Pi's. I started with Kingston cards that turned out to be slightly larger than PNY and I couldn't copy my original images until I shrank the image and let it expand when it was first used.

I won't buy any high capacity cards for applications where data is written frequently; like Raspberry Pi's because they go through cards quickly. I've had half a dozen Kingston and PNY cards become write protected after 6-12 months. The cards fail in that mode so, in theory, you can get files off of a defective card. That hasn't been the case for some of my bad cards because Linux wouldn't mount the bad partitions.

 

I do Understand that Hard Drives and SD Cards all need a Files System.

And I do know that if we Say 1. MB it will be a little Off when a Computer Counts because a Computer Starts Counting at 0. so I get all this.

So let Me Ask this?

I have a 256 GB Micro SD Card and when I go to Format it at the Top it Says 250 GB and I get this.
It is Telling Me that when I Format this Mocro SD Card it will have 250 GB of space on it because the Computer is always a little Off then how we Count.

So I can be ok with 250 GB but after it Formats the Micro SD Card it goes Down to 249 GB what is this?

 

I do Understand that Hard Drives and SD Cards all need a Files System.

And I do know that if we Say 1. MB it will be a little Off when a Computer Counts because a Computer Starts Counting at 0. so I get all this.

So let Me Ask this?

I have a 256 GB Micro SD Card and when I go to Format it at the Top it Says 250 GB and I get this.
It is Telling Me that when I Format this Mocro SD Card it will have 250 GB of space on it because the Computer is always a little Off then how we Count.

So I can be ok with 250 GB but after it Formats the Micro SD Card it goes Down to 249 GB what is this?

What we have already talked about. Formatting and file systems require multiple sources of overhead, the details of which may or may not be apparent to users.

 

Ok I have another 256 Micro SD Card and I put it in and go to Format it.
And at the Top it Says 233 GB Capacity.

And this is Before I Select any FAT Size or do anything.

 

I have an 8GB microSD card on my STM32F system and I get 100% of 8GB.
That is because I do not use a file system. I read and write directly to raw data blocks.

 

Ok I have another 256 Micro SD Card and I put it in and go to Format it.
And at the Top it Says 233 GB Capacity.

And this is Before I Select any FAT Size or do anything.

OK, so the basic overhead is 9%