Sandisk Standard SD Card 2GB Datasheet

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:

Sandisk products:

Operating systems:

Hardware platform:

Description:

Secure Digital

All Microsoft

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)."