Symptoms:
Why is the capacity of my Secure Digital memory card (as reported by many operating systems)
different than the capacity that is listed on its label?
Affects:
Description:
Sandisk products:
Secure Digital
Operating systems:
All Microsoft
Hardware platform:
All PCs, Jornadas, etc.
Cause:
The operating system, when reading the size of the card, reports a slightly different capacity than what is
listed on the card’s label. SD has a small overhead for the security features.
Solution:
Definitions of a Megabyte:
1) Operating Systems commonly define a Megabyte (MB) as: 2 to the 20th power (1,024KB--Kilobytes).
2) DiskDrive and Flash Memory Card Manufacturers commonly define a MB as one million bytes (exactly
1,000,000 bytes).
Unformatted (Capacity)
Also known as drive byte capacity before formatting. The Maximum capacity of disk drive before formatting
equals
[ (# Cylinders) X (# Heads) X (# Sectors) X (# Bytes per Track) ]
Example:
64MB CompactFlash Card consists of:
490 Cylinders
8 Heads
32 Sectors
512 Bytes per Track
This equates to: [ (490) X (8) X (32) X (512) ] = 64,225,280
Unformatted Capacity: 64,225,280 bytes
Formatted Capacity: 63,934,464 bytes (User Data)
Cause:
Disk Drive Companies such as SanDisk define 1 MEGABYTE as 1,000,000 BYTES. Operating Systems
define 1 MEGABYTE as 1,048,576 BYTES (1024K X 1024K or 2 to the 20th power).
Example:
SanDisk 64MB CompactFlash Card being read by Microsoft Operating System.
SanDisk Total Formatted Capacity divided by 1 MB (as defined by the Operating System) equates to the
following: 63,934,464 BYTES / 1,048,576 BYTES = 60,972,656 BYTES, 60.9MB displayed by OS.
Security overhead
SD-8
"The total card capacity of 8 million bytes is divided into a security area and a user accessible area (of
approximately 6.7 million bytes)."
SD-16
"The total card capacity of 16 million bytes is divided into a security area and a user accessible area (of
approximately 14.6 million bytes)."
SD-32
"The total card capacity of 32.1 million bytes is divided into a security area and a user accessible area (of
approximately 30.5 million bytes)."
SD-64
"The total card capacity of 64.2 million bytes is divided into a security area and a user accessible area (of
approximately 62.3 million bytes)."
SD-128
"The total card capacity of 128.4 million bytes is divided into a security area and a user accessible area (of
approximately 125.8 million bytes)."
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