There are different speed grades available, measured the same as CD-ROMs, in multiples of 150 kB/s (1x = 150 kB/s). Basic cards transfer data up to six times (6x) the data rate of the standard CD-ROM speed (900 kB/s vs. 150 kB/s). High-Capacity cards are made with higher data transfer rates like 66x (10 MB/s), and high-end cards have speeds of 200x or higher. SanDisk classifies their cards as:
Ultra II — minimum read speed of 15 MByte/s (100x)
Extreme III — maximum speed of 30 MByte/s (200x) (SanDisk Class 6)
Extreme IV — up to 45 MByte/s (300x) (Currently only available for Compact Flash formats)
Note that maximum read speed and maximum write speed may be different. Maximum write speed typically is lower than maximum read speed. Some digital cameras require high-speed cards (write speed) to record video smoothly or capture multiple still photographs in rapid succession. This requires a certain sustained speed, or the video stops recording. For recording, a high maximum speed with a low sustained speed is no better than a low speed card. The 2.0 specification defines speeds up to 200x.
SD Cards and SDHC Cards have Speed Class Ratings defined by the SD Association. The SD Speed Class Ratings specify the following minimum write speeds based on "the best fragmented state where no memory unit is occupied":[8]
Class 2: 2 MByte/s - 13x
Class 4: 4 MByte/s - 26x
Class 6: 6 MByte/s - 40x
SD and SDHC cards will often also advertise a maximum speed (such as 133x or 150x) in addition to this minimum Speed Class Rating. Important differences between the Speed Class and the traditional "X" speed ratings are; 1) the ability of the host device to query the SD card for the speed class and determine the best location to store data that meets the performance required, 2) class speed defines the minimum transfer speed. Even though the class ratings are defined by a governing body, like "X" speed ratings, class speed ratings are quoted by the manufacturers but unverified by any independent evaluation process.
On 21 May 2009, Panasonic announced new "class 10" SDHC cards, claiming that this new class is "part of SD Card Specification Ver.3.0".[9] Toshiba also announced cards based on the new 3.0 spec[10] As of November 2009[update], the SD Association's Web site does not include information on this new class or new specification.
Glad I got that sorted, all clear now!