Why a hard drive or flash drive has less storage space than promised ?

It has happened to most of us. We buy a new hard drive (or maybe a flash drive) without expecting that the storage capacity will be less space than what was mentioned on the box and we start cursing the manufacturer and our dealer for false marketing thinking that they should be sued for doing this. Hey, but have you ever wondered how they continue to do this again and again without getting into legal trouble ?

Your hard drive may be reporting a smaller number for a few reasons, but the first thing you should know is that the number displayed is completely normal. Here’s why.

1. External Drives Often Come With Software Installed

The first (but less important) explanation is that many external drives come with software from the manufacturer, like backup software or something similar. Generally, we recommend getting rid of this, as it’s rarely better than the stuff you can download online. So, while it won’t free up a ton of space, you should probably format that drive before you do anything else.

If you’re a Windows user, just right-click on the drive in Windows Explorer and choose “Format”. Format it to NTFS if you’re planning on storing a lot of data (like music or a backup), and FAT32 or ExFAT if you plan on using it to transport files between Macs and Windows PCs.

If you’re a Mac user, go to /Applications/Utilities/Disk Utility, and click on the drive in the left sidebar. Click the “Erase” tab in the middle pane, then choose your file system from the dropdown. Mac OS Extended (Journaled) is the best default option for storing a lot of data, but FAT32 and ExFAT are better if you’re using it on Mac and Windows PCs.

Once you’ve formatted it, you won’t have those annoying software popups when you plug it in, and you should have a bit of extra free space on the drive.

2. Hard Drives and Flash Drives Are Measured One Way, But Marketed Another

Another explanation is that they are not marketing it falsely at all. The explanation is that a manufacturer considers 1 Megabyte to be 1000 Kilobytes, 1 Gigabyte to be 1000 Megabytes, 1 Terabyte to be 1000 Gigabytes and so on. This is correct considering that kilo means 1000 and mega means 1000000 (10^6). However, computers calculate on base 2 and to them, 1 MB is actually 1024 kilobytes, 1GB is 1024MB and 1 TB is 1024GB. This difference in the method of computation is responsible for this "missing space."

Lets take an example of a 500 GB hard disk.

From a manufacturer’s point of view, the 500GB will have 500*1000*1000*1000 = 500000000000 bytes.

From a computer’s point of view, 500GB is actually 500*1024*1024*1024 = 536870912000 bytes.

So, a hard drive that promises to have 500 GB storage space will actually display 465.66GB, 536870912000-500000000000 = 36870912000 bytes (34.34GB) less storage space when connected to a computer.

Space Promised Displayed on a computerDifference
100GB 93.13GB 6.87GB
250GB 232.83GB 17.17GB
500GB 465.66GB 34.34GB
1TB 931.32GB 92.68GB
2TB 1862.64GB 185.36GB

Take a look at the table given above to see how much space is "lost" due to computers working on a base 2 system. As you can see, with the increase in capacity of the storage device, there is an increase in the missing space.

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