Picture the scene, you have just bought a new 1TB hard drive, installed it into your computer, fired it up, formatted it and see that it’s only 0.9TB in size. What happened to the rest? Have you lost data or is the hard drive broken?
This is a common question we hear at Dave’s Computers in New Jersey. Very common in fact. Whenever we install new hard drives for customers and fire up their computer to show them, 9 times out of 10 we are asked why it’s smaller than what we said we installed.
The hard drive is exactly the size we said it was but the way the operating system reports it is different. You’ll have to bear with me as there is some math involved.
Why do advertised hard drive capacities appear smaller than expected?
Hard drive capacities appear smaller than advertised primarily due to differences in how hardware manufacturers (who use base 1000 bytes for 1TB) and operating systems like Windows (which measure in multiples of 1024 or 512 bytes) calculate storage. Additionally, space is consumed by the file system and essential data, similar to how bookshelves reduce space in a library. For example, a 1TB hard drive will have the capacity to store 1,000,000,000,000 bytes, but once formatted in Windows, it will show much less because the operating system needs to allocate space for its file system and features like Windows Shadow File System (which can take 5-10GB). Therefore, you haven’t lost any data; it's simply a difference in calculation and system overhead.
1TB, or terabyte equals 1,000,000,000,000 bytes. That’s a trillion bytes. It will have somewhere in the region of 1,953,525,168 sectors of 512 bytes each. With the help of my trusty calculator, that gives us 1,000,204,886,016 bytes of storage. Slightly above the advertised capacity.
When you format it in Windows, you won’t get those numbers. The reason is that round numbers work fine in hardware but don’t work in software. A number has to be equally divisible in software programming to make it work. So where 1TB is a nice round number to hard drive manufacturers, it isn’t divisible by that 512 byte size.
Windows will instead measure the drive in multiples of 1024 or 512 which will result in a different size. Then you have to add the format data to that.
Think of a file system like a library. The building itself could be large but once you put the bookshelves in, the building looks a lot smaller. The same principle applies to a hard drive. Once Windows loads its file system, the available space is reduced. Then add Windows Shadow File System which is like a secure copy of files and even more space has gone.
So you haven’t lost any data and you haven’t been robbed. It’s just the differences in how hardware manufacturers and Apple and Microsoft calculate size. Then add the file system and essential data to come up with available space.
If you want to try it yourself, open a CMD window in Windows. Select your new hard drive by typing ‘D:’ or whatever and hit Enter. Type ‘vssadmin list shadowstorage’ and hit Enter.
You will see Shadow Copy Storage space of 5-10GB. This is some of the shortfall in space and why Windows says your disk is smaller than it actually is.