Summary: FujiFilm, Olympus and Toshiba share licensing, but only Toshiba actually manufacture all XD media.
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Look at the CHIP, rather than its packaging: there are differences, for example, between FUJI and OLYMPUS XD media. One of these differences is that ONLY the Olympus brand XD cards are guaranteed capable of what is called, "Olympus Panoramic Mode". For this reason, Olympus brand XD media are often sold at a small premium to FujiFilm brand XD media. However, anecdotal evidence gleaned from various NewsGroups, Digital Camera Forums, etc. suggest that BOTH Olympus and FujiFilm XD-512Mb media work, fully-featured, in FujiFilm cameras.
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There are also differences in performance & compatibility between different SIZES of XD media. For example, Fuji technical docs state that their "card readers" (or "holders") are only specified to work up to XD-512Mb size. There are scattered reports that some XD-1Gb owners have experienced compatibility problems, whereas ALL XD-512Mb media work well when connected directly to PC's and laptops.
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And it is interesting to note that some TIMING TESTS show that XD-512 media are FASTER than XD-1Gb. This performance difference exists, apparently, because of a difference between XD-512Mb and XD-M1Gb : note the "M"... this "M" means that the 1Gb size uses a different (more dense) way of storing bits on the XD media.
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Indeed, XD-M512Mb "Type M" media have begun to appear; if nano-second differences in recording speed, downloading -to- PC speed, etc., are important to you, ensure that your XD-512 media is NOT type "M".
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Finally, when we look at the pricing structure of XD media, XD-M1Gb at say ~$80.00 USD looks like a no-brainer when compared to XD-512Mb at ~$60.00; indeed, if your primary purpose is to use your storage media for motion/video, then you really must look at larger capacity. On the other hand, if your interest is simply digital photography, one XD-512Mb card is the optimum size.
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That 512Mb, at 3/4 the price of 1-Gb media is not more closely priced to 1/2 the price of 1Gb media ~ probably reflects the much higher demand for "optimum" Capacity, Performance & Compatibility, rather than (sheer bit density) size alone.