HDD Disk Space used by File allocation table

February 27, 2013 at 01:04:38
Specs: Windows XP / Windows
Hard Drive Space ..... The other thing you have to look at here is the fact that the FAT "File allocation table" has to take up hard drive space also ... so that the hard drive knows "Maps" where everything is on the hard drive ... this also takes up hard drive space .. so for a 80Gb Hdd you would probable be looking at around 73gb usable space that you can store stuff on ..... The larger the hard drive, the more FAT will be taken from your total hard drive size, as there is more info on a higher capacity hard drive, so therefore more FAT taken from the total hard drive size .. !!!

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February 27, 2013 at 04:48:23
The larger the hard drive, the more FAT will be taken from your total hard drive size,

Not true. The File Allocation Table is the same size regardless of the disk size from the smallest to the largest.

The thing that does change with larger disks is the cluster size and it is this that mikes large FAT formatted drives uneconomical.

The reason for this is that each entry in the FAT indicates whether a cluster is in use or not. Every FAT32 formatted disk has the same number of clusters. Bigger hard disks have bigger clusters in order to fill the available space..


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February 27, 2013 at 05:03:06
The main reason for the manufacturer's stated drive capacities being higher than what they actually are in the "real" world is that they use the decimal system and no manufacturer wants to be the first one to change that.

Most experienced computer geeks know that 1 megabyte = 1024 kilobytes.

The drive manufacturers, however, continue to base their stated capacities on
the decimal system: 1 megabyte = 1000 kilobytes.

The greater the capacity, the greater the difference will be between the stated and the "true" capacity.

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February 27, 2013 at 05:12:34
phill22 is correct. The difference between advertised capacity & true capacity is due to the 1000 vs 1024 issue. It has nothing to do with FAT.

"so for a 80Gb Hdd you would probable be looking at around 73gb"

Incorrect. Advertised capacity x 0.931 = approx actual capacity, so an 80GB HDD would be 80 x 0.931 = 74.48GB (approx).

Or you can use the HDD capacity calculator at the bottom of this page:


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February 27, 2013 at 07:37:15
Removed post - I was thinking about MFT not FAT.

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February 27, 2013 at 08:31:34
I find this whole thread somewhat irrelevant - who uses FAT nowadays? But if we are going to criticize it, let's get it right. The OP is correct to say that the size of the File Allocation Table varies depending upon the size of the hard disk. It would be a pretty stupid file system (well, it is, but let's overlook that) if it relied just on the rather coarse-grained factor of cluster size. Cluster size is more important when determining the maximum size of a partition; double the cluster size and you double the number of sectors - and hence the size of disk - that a FAT entry can represent. Use that same cluster size with a smaller disk and you can get away with a smaller FAT table; but you waste more space on the disk as files must always occupy an integral number of clusters.

There actually is a reason for that field in the boot sector that tells you how many sectors are occupied by the FAT - the reason is that it is not the same for all hard disks/partitions. If you don't believe me, try writing a program to access a FAT filesystem.

And, of course, it is also true that this metadata reduces the amount of storage available for data storage in any filesystem. So while the OP was wrong to ignore the 1000/1024 factor he was quite right to say that the metadata (not just the FAT) reduces the capacity; that largely accounts for the difference between riider's 74.48GB and the OP's 73GB.

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February 27, 2013 at 10:54:07
Maybe I misinterpretted the OP's statement, but I took it to mean the reason an 80GB isn't 80GB & is closer to 73GB is because the FAT table takes up the space. He made no mention that he's aware of the 1000/1024 issue. Even so (& I'm no expert when it comes to the behind the scenes workings of a file system), I find it hard to believe a FAT table would take up approx 1.5GB (74.48 -73) on an 80GB HDD.

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February 27, 2013 at 11:14:03
Without actually working it out, I'd guess something like 300MB per FAT table. Don't forget there are two of them, so that's 600MB. And the space at the start of a disk isn't all used, so that's a bit more. The boot sector doesn't take up much space, but there's the root directory. And on a real filesystem all the directory entries are what I would call metadata - they can't be used to store real data. It all adds up.

My real beef with the replies in this thread was the first one, which mistakenly "corrected" somthing the OP said. Fact is the OP was right and the correction was wrong. I think it's a bit rough on a new member to treat them this way.

But FAT is boring. Ext3 is far more fun.

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February 28, 2013 at 21:48:57
NTFS also uses a lot of space to store the meta data.

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