Can I use the Espruino to emulate a MicroSD card?

Can I use the Espruino to emulate a SD memory card?

I.e., I plug in a microSD (via some kit hopefully even more useful than http://www.ebay.com/itm/TF-Micro-SD-To-SD-Card-Extension-Cable-Adapter-Flexible-Extender-For-Car-GPS-YG-/112084033724 ) to the GPIO pins, and the Espruino will emulate a block device i.e. block-read and block-write commands.

What speed would I get?

Afaik, Espruino can currently only be SPI master and not slave... because slaves have to follow pretty narrow timing specs... I'm recalling some forum entry/entries talking about that.

@Gordon, is this still the case?

1)
Just for a reference of someone who did at least close to this:

http://webpages.uncc.edu/~jmconrad/ECGR6185-2008-01/notes/SD_CARD_DISPLAY_MICROCONTROLLER.pdf these guys did it.

Code here ftp://ftp.circuitcellar.com/pub/Circuit_Cellar/2007/209/Davaine-209.zip .

Discussed here https://www.raspberrypi.org/forums/viewtopic.php?f=44&t=135915 .

2)
http://electronics.stackexchange.com/questions/29010/sd-card-emulation even suggests that SPI wouldn't be enough but that modern SD cards go "25Mhz".. what's the issue?

How would you implement this?

Also any more references on this forum would be much appreciated.

Em, maybe this one is on this topic too, not sure: https://hackaday.io/project/6435-sd-card-sized-board-lets-say-sduino .

I guess I see what you want to do... My question is then: what are you expecting from Espruino to do? Because the SD Card API is basically directory and file handling (by your SD card hosting device). Furthermore to be aware of is the fact that the SD Card is a single port memory... and this allows any device accessing it to cache some of the date... Now one can argue, that a camera does the same... but not exactly... The camera is not active on the SD card while connected to the computer and the connection is not via SD card interface but mostly USB (old days: plain serial).

SDIO is like SPI but with 4 data wires instead of 1. Basically all SD cards can also work as Multi-media cards (MMC) - it's an old standard but is easy to use and is open, so it's what we use on Espruino.

As far as I know the higher speed SDIO standard is proprietary - you're supposed to have a license to use it, and I don't believe it is that well documented unless you pay.

Realistically that's what most devices that use SD cards will expect though, so it's what you'd have to implement. Some of ST's microcontroller chips do have an SDIO peripheral on them which you may be able to use (I'm not sure if Espruino's do...), but it'd not something you could do from JavaScript - you'd have to get into some pretty heavy C programming for that.