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Between a SD card and a Compact Flash card for using in camera, which I write RAW images of 40 mega byes on it, sometimes in burst mode and never for video, Is there any technical reason to favor SD cards in general to Compact Flash cards for "Reliability" and vise vera. I understood that it depends on Manufacturer, etc.. imagine the best SD card of SanDisk vs their own best CF card! I mean actually comparing the electronics of a SD card vs a CF card to know which one I should trust for reliability.

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In general the top tier Compact Flash cards available to the typical consumer via retail channels when compared to the top tier Secure Digital cards available to the typical consumer via retail channels are:

  • More rigid and less susceptible to damage from flexing
  • Rated for more extreme environmental conditions (higher altitude, lower temperature, etc.)
  • Have faster write speeds (although this gap has been narrowing recently), even when read speeds are the same or very similar
  • More expensive

Additionally, CF cards use the standard IDE protocol to communicate with other devices. SD cards use their own protocol that has been modified/extended several times (e.g. SD → SDHC → SDXC → UHS → SDIO). Device compatibility with SD cards newer than the device can be an issue. Newer, faster CF cards tend to be more backwards compatible with older devices, albeit at the older device's speed rating.

SD cards only have enough connectors to move between one bit (original SD standard) and 8 bits/one byte at a time (only the fastest, most advanced UHS-I or higher cards). CF cards have 50 connector pins and enable multi byte parallel write or read at the same time. Your camera must support multiple port parallel write to take advantage of this difference.

On the other hand, the pins on devices that use CF cards can be bent while the connection between SD cards and their compatible devices are less susceptible to damage from card insertion and removal.


Appendix (probably obsolete by the time it is typed) Current fastest transfer rates as of August 24, 2015 as specified and available on amazon.com

SD cards: SanDisk-Extreme-UHS-I-Memory-SDSDXPA-032G-AFFP Capacity: 32GB Protocol:SDHC I, Class 10, U3 Speed: Up to 95MB/s Cost: $29.95

CF Cards: SanDisk-Extreme-CompactFlash-Memory-SDCFXPS-032G-X46 Capacity: 32GB Protocol: UDMA-7, VPG 65 Speed: Up to 160MB/s Cost: $59.95

  • Did you mean to write that SD cards move one bit at a time, not byte? CF cards can move up to 16-bits (2-bytes) at once but have to run to a much (much) slower clock. – James Snell Aug 24 '15 at 10:45
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    1 bit SD (or actually MMC) was true long time ago, it is 4 bits now since 2006 (SDHC, SDXC) – szulat Aug 24 '15 at 12:04
  • Re: current SD cards: Double data rate operation at 50 MHz (DDR50) is also specified in Version 3.01, and is mandatory for microSDHC and microSDXC cards labeled as UHS-I. In this mode, four bits are transferred when the clock signal rises and another four bits when it falls, transferring an entire byte on each full clock cycle. – Michael C Aug 24 '15 at 20:00
  • @szulat - could quibble over semantics but it's not going to help clarify the answer, SD Specs before v4.0 (UHS-II) have one physical data line so it's still one bit at a time, just modulated faster. UHS-II has 4 data lines and in the case of the SanDisk SDSDXPB-016G-G46 Extreme PRO a read speed of 280MB/s. CFast (if we're going there) is even quicker if memory serves. Not quite sure that it addresses anything to do with reliability though. :) – James Snell Aug 24 '15 at 21:32
  • Hmm. That card is not available via amazon in the U.S. – Michael C Aug 24 '15 at 21:40
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All flash memory systems have their physical enclosure and 2 electronic systems - a controller and flash memory to determine reliability.

Electronics

Electronically there is no discernible difference because the storage side of flash memory (NAND) is the same across all the types (CF, SD, etc.) NAND at the chip level has a serial interface and relies on controllers to make it usable, manufacturer implementations differ but the principle has remained the same since it was invented in the 80's.

Making 100% reliable NAND is commercially impossible, so between the flash chip(s) and the device (camera etc.) we have to have an interface. Electronically this interface is a single point of failure on all form factors - provided your connection is electronically compatible (it sends the right signals) then again the controllers are implemented in silicon and are equally reliable.

As far as data reliability goes, any differences are purely down to production quality and how the manufacturer has implemented the workarounds you need for NAND like wear-levelling, error correction, and bad block detection/marking in the interface.

Packaging

The physical form factor is really a toss-up between pins vs. edge connectors.

CF's greater size does allow for more physical protection and rigidity but this could be addressed in the materials of SD cards if there were demand for it. Industrial SD cards rated to handle operating temperatures of -40 to 85 Celsius which would outdo most cameras are available. Properly secured both are able to withstand shock loads beyond most cameras.

CF has more pins which could improve it's performance under vibration loads but the majority of the pins are throwbacks to having controllers outside the device and the additional data lines (8/16-bits vs. 1-bit) offers no additional resilience.

I have seen casings on SD cards fail due to poor materials but this hasn't been the design of the interface at fault - the cards tend to still work long enough for data to be recovered and they are less likely to damage delicate pins in the host.

Anecdotally, where SD casings have failed on me the data was still accessible - CF cards I've not been so lucky.

TL;DR

Electronically there's nothing in it. Knowing quite a few ham-fisted types I tend to favour SD's edge connectors as being more robust. But you'd be better off getting whatever body offers the features you want and not worrying about media reliability.

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