It’s a common
question, “Why do I have less space on my hard drive than I thought?” Now that
multi-terabyte drives are common, this question may not seem important to some
but the increased use of SSDs and partitioned drives makes it very relevant.
Here are some answers.
Different
ways of measuring disk size
To begin with,
you may start out with the perception of less space than you thought because of
the difference between the way that manufacturers label disk sizes and the way
that computers measure size. It has to do with decimal-based numbers versus
binary-based numbers. Disk manufacturers use the system 1 kilobyte=1000 bytes.
Computer systems use 1 kilobyte=1024 bytes. The discrepancy between the two
systems grows with disk size. A 500 GB drive will be read by the operating
system to have only about 466 GB. Of course, the bytes haven’t gone anywhere. A
byte is still a byte but the different ways of measuring size can cause
confusion.
Perceptions
aside, there are many real ways that disk space gets used up even when
you don’t put any files of your own on a disk.
NTFS
file system overhead
First of all,
Windows systems likely have the file system NTFS and this file system comes
with overhead. It stores file information in something called the Master File
Table (MFT), which in its default configuration can occupy as much as 12.5% of
free disk space. The amount that MFT actually uses varies according to your
system and usage patterns. Examples of MFT sizes that I have seen range
from a few hundred MB to several GB.
Hidden
partitions
In newer
systems with Windows 7 or 8, there is likely a hidden partition called the
System Reserved Partition. It has no letter and is there to support BitLocker
encryption, the boot configuration database, and the Windows Recovery
Environment. In Windows 7 it is 100 MB and in Windows 8 it is 350 MB. You can
read more about it at this link.
Most PCs also
come with a hidden recovery partition that holds a Windows image to be used as
a backup for reinstalling the original OEM setup. This partition is typically
upwards of 10 GB or more.
Windows
system files
Then there is
the operating system - about 10 GB for Windows 7 or 8. But along with the
operating system itself, there are a number of accompanying space consumers.
Here are some hidden system files to look out for.
- There will be a page file whose size usually is 1 to 3 times the size of the installed RAM. It can be adjusted. See this Microsoft reference
- If you use hibernation, the Hiberfils.sys file may occupy space equal to size of the installed RAM.
- If you have upgraded the operating system, there may be a large backup. For example, see this article.
- System Restore points – Typically, 3 to 5 percent of the disk but at least 300 MB. Can be adjusted, See this Microsoft reference.
It is also
worth noting a related situation. It isn't uncommon for a laptop to come with a
hard drive partitioned into a C: system volume and a D: data volume with the C:
partition being rather small. In this type of setup, there will not be a lot of
free space on the C: drive.
And there you
have it - some ways your disk space gets used up before you even start.
