What’s the Difference Between a CompactFlash (CF) Card and a Microdrive?…
...And with CF cards, do the stated speeds of 40x, 80x, etc. work with all cameras?
A: Microdrives are miniature hard drives whereas CompactFlash (CF) cards are solid-state microchips (flash memory). Microdrives use the thicker Type II CompactFlash format, so they fit in the same sized housing and have all the same connections as CF cards. Their moving parts require more power than flash memory and they're more fragile than CF cards. As long as your camera has a CF Type II slot, you can use Microdrives, but check your manual to be sure your camera supports them.
[Update]: Microdrives are no longer manufactured having been surpassed by the rapid development of sold-state memory cards. CompactFlash has also fallen out of favor now as larger capacity SD, Mini SD and Micro SD cards which are physically smaller are favored by camera manufacturers. Mini/Micro SD cards are also cheaper than CompactFlash cards.
The 133x, 800x, etc. speed rating on memory cards< is based on the data transfer speed that the card will support. The 1x figure of 150Kb per sec comes from the transfer speeds quoted on CD drives, so an 800x card is 800 times faster than this (with a transfer speed of about 118 Mb per sec). It doesn't mean it's 800x faster than a standard card though, as that may already be a 4x or 8x card in the first place. One factor to consider is how fast your camera itself can transfer data. If you have an entry-level model, there's not much point in using the latest, super-fast card. Match the speed of the card to your camera - slowish cards for compact cameras, medium speeds (up to 800x) for high-end compacts and budget SLRs and fast cards (1000x plus) for high-end SLRs.
[Update]: Older Canon EOS cameras like the EOS 300D and 350D used Compact Flash memory cards so be aware of this if you're considering picking one of these on the used-camera market.
Memory Card Videos:
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