Why My Storage Device Has Less Capacity Or Speed Than Expected

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When you buy an external storage device, you may encounter some issues with the capacity or read/write speeds of the device. Whether it is a micro SD card, external SSD drive, or HDD drive or a pen drive. Below are the most common issues.

  • Lower storage capacity than the advertised one
  • Lower data transfer speed than it is mentioned in specs
  • Connectivity issues

In this article, I am going to explain some of these common issues and possible solutions for them. I have done thorough research and testing and I am going to explain my findings in this article.

Let’s go through each one.

Lower Storage Capacity Than The Advertised Capacity

This one is quite common and a lot of people face this issue when buying an external storage device. There are some reasons for this problem. And each one requires its specific solution.

Difference Between Advertised Capacity And Actual Capacity

The standard units of storage are base two numbers, like 1 KB which is 210. That equals 1024 bytes. 1MB equals 220 which is 1,048,576 bytes, and so on.

But, all vendors use decimal units to express the capacity and transfer speeds of their storage devices.

The decimal units would be:

  • 1 KB equals 1000 bytes
  • 1 MB equals 1,000,000 bytes
  • 1 GB equals 1,000,000,000 bytes.

Note that these units are smaller than the base two units. The decimal 1KB unit is 1000 bytes which is less than the standard 1 KB which is 1024 bytes.

When you plug a device to your PC or mobile phone, the units are measured in base two units. Thus there would be a difference in capacity and transfer speeds than the advertised numbers.

For example a 500GB advertised capacity would become 500,000,000,000/(1024x1024x1024) = 465GB. Also, the file system data takes some space and there may be some hidden system files. Thus the usable capacity of the device, even when totally free, would be lower than 465GB.

In general, the advertised capacity of ANY storage device will translate to a lower capacity when connected to a PC or a mobile or a tablet. Whether it is an internal or external device, and from ANY vendor.

The following table shows common advertised capacities and their equivalent actual capacities.

Advertised CapacityUnitActual CapacityUnit

The same is true for data transfer speeds. For example a 100MB/s (megabytes per second) read speed would translate to 100,000,000/(1024×1024) = 95MB/s.

Keep in mind that it is common to all vendors and all storage devices, whether it is internal or external. And you can’t do anything about it. The devices are manufactured in this way and couldn’t be changed.

Incorrect Drive Partitioning

All storage devices need partitioning before they could be used with a computer or a mobile/tablet. During partitioning all or a part of the space is allocated to a partition which is known as drive or disk drive.

In some cases, the devices are partitioned incorrectly. For example, a 64 GB pen drive has a partition of 20 GB and the rest of the space is raw space or unallocated space. The drive still has 64 GB capacity but the computer or tablet will only show 20 GB capacity.

Unfortunately, most of the file managers don’t show unallocated space. Unless you are using some disk partitioning tools, you will not be able to see the unallocated space. And it may seem to you that the drive has less storage than it should have.

Let’s go through an example. I bought a 2 GB pen drive and it showed 1 GB capacity. When I checked in Disk Management (on Windows 8.1) there was around 1 GB unallocated space.

disk management showing unallocated space of a pen drive

I used diskpart, which is a command line utility in Windows and deleted the partition. Then I created a new partition allocating all the space to it.

The picture shows how I deleted the partition. If you want to use it, make sure you select the right disk, otherwise, you will delete your computer’s drives and will lose important data.

deleting a partition using diskpart utility.

And finally the drive has expected capacity.

creating new partition using diskpart utility.

Also, there are free and paid partition managers to make this task easier and safer for you. I personally used EaseUS Partition Master and it worked perfectly. It is free and provides everything you need. But it only installs on Windows.

You can also use GParted for partitioning, which works on Linux, Windows, and Mac OS. And it is free.

Fake Storage Device

Sometimes when you buy an external storage device such as a micro SD card or a pen drive, you receive a fake product. Some of the sellers will sell very low quality or even totally fake device, claiming the device is from a trusted vendor.

This happens when you buy from Amazon or other eCommerce platforms. Even if the seller is SanDisk or Samsung, the product could be fake.

The solution is to check the device to make sure it is genuine. And if you find that the device is fake you should return it. This should be the first thing you do with a newly purchased storage device. Test the device within the return and replacement period.

You can use some software applications to test the device and make sure it is not fake.

Samsung has its own software like Memory Card/UDF Authentication Utility. You can download it from official Samsung website.

There are other software applications like FakeFlashTest, and H2testw (very slow but performs a thorough test) on the PC, or some Android Apps.

These later software tools help you test both capacity and read/write speeds.

If you find out that the actual capacity of a storage device or its transfer speed is much lower than the given specs, you should return the device and ask for a replacement or refund.

Lower Actual Data Transfer Speed

This is another common issue with external storage devices. As mentioned before device manufacturers use decimal units to express the speed of their devices. But computers and tablets use binary units to calculate the speed and capacity.

For example 1 MB binary unit will be 1,048,576 bytes, whereas 1 MB decimal unit will be 1,000,000 bytes. The decimal unit is a little bit smaller than the binary unit. So the speed when calculated by your computer or tablet will be lower than advertised speed.

A 100 MB/s decimal speed will translate to 100,000,000 / (1024×1024) = 95MB/s binary speed.

The following table shows common advertised speeds and their equivalent actual speeds.

Advertised SpeedUnitActual SpeedUnit

The read/write speeds are measured under specific conditions, like using the latest PC model, using a standard connector cable, and no file fragmentation. In practice, some of these conditions will not apply and the transfer speeds would be much lower than the advertised speeds.

External Storage Device Connectivity Limitations

Did you know that every port on your computer or mobile device has speed limitation? Probably not. If you want a flash drive to have a speed of 700 MB/s and it is USB 3.0, that speed would be impossible. Because USB 3.0 does not support a speed of 700 MB/s.

Note that USB interface is different from USB port type.

All USB interfaces have speed limitations. Below are the USB interfaces and their maximum speeds.

  • The maximum speed of USB 3.0 Gen 2 is 625 MB/s
  • The maximum speed of USB 3.1 Gen 2 is 1250 MB/s
  • The maximum speed of USB 3.2 Gen 2×2 is 2,424 MB/s

The only option for ultra-high-speed external storage devices is to use Thunderbolt 3. It supports transfer speeds up to 5 GB/s. But then there is another limitation. Not all computers and mobiles/tablets support Thunderbolt 3. Such a device could be used with a limited number of computers or tablets.

To cope with speed limitation of ports, some storage devices support all USB interfaces. For such devices the transfer speed would vary based on the port being connected to. When connected to USB 3.0 the max transfer speed would be around 600 MB/s. And when connected to USB 3.1 the max speed would be around 1200 MB/s.

That is one reason you don’t get the high transfer speed of an external storage device. It is connectivity limitation.

Also, connection cables can play a role. Non-standard cables will not be able to transfer all the data sent by the device. This is true, especially for ultra-high-speed external storage devices. You should use the standard cable that comes with the device.


In this article, I explained the most common issues with capacity and read/write speeds of data storage devices. Unless you buy a fake device, which you should immediately return, there is always a solution to your problem.

If you have any other issue with the capacity or speed of your data storage device, leave a comment below. I would be happy to address that issue and propose a solution.

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