I have read somewhere that I should not use a Micro-SD card (with an adapter) for the main storage & OS boot of my Raspberry Pi. Is it true? Why?
In some cases, the signal integrity of SD cards can be marginal - the signals have to come off the SoC, down the PCB traces, through the socket, and into the SD card. SD cards, especially fast cards, can be clocked at up to 100MHz. MicroSD adaptors are often designed to be as cheap as possible, and may make the signal integrity worse. For example, I have a £1 USB card reader that will read MicroSD in its own dedicated MicroSD slot, but not in an adaptor in the normal SD slot (which reads full SD cards just fine). If you open up an adaptor, you find it's just a series of metal pins in plastic - no shielding or any other signal protection.
Now the Pi isn't a £1 card reader, but the same issues may apply - the more connectors you have, the higher chance you have of pushing it over the edge in high speed modes. If you look at dmesg and see SD command errors, that's where you'll see problems if they occur. It's more likely if you're thrashing the card (eg using as swap) rather than people who just use them for moving photos about.
In general, if someone states that, he should also provide some arguments about his claim. Otherwise, you should ignore such statement.
I am using such adapter without problems and I really have no idea why it could be wrong. The only thing I imagine is that many such adapters are not really good quality and you can easily break one.
I've been using a micro sd card in an adapter for the past few months and have had no problems. There are even adapters that fit flush with the side of the Pi so there is no bit 'sticking out'. It's fine to use a micro.
Two adapters available: US-based: http://www.adafruit.com/products/966
There's no reason why this should be true. As far as the Pi is concerned, an SD card and a MicroSD card + adapter are exactly the same thing.
I could imagine however that MicroSD cards may not perform favorably compared to a full-size SD card and in this respect it might not be a brilliant idea, but this is definitely not a "should not" and more of a "you'd be better off if you didn't" kind of thing.
The full size, standard SD cards came first... As products that use this media became smaller,for example action sports camera's and suchlike..,then the SD card required, also became smaller...The adapter just means that you can use both types in older or different types of devices, or in your computers, or Standard SD ( full size card readers ) Just my opinion anyway........