Transcript
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)."
