Removable storage is problematic when it comes to reliability. Simply powering the device off while it is writing will cause issues, worse is when you have vibr

Removable storage is problematic when it comes to reliability. Simply powering the device off while it is writing will cause issues, worse is when you have vibration momentarily overcoming the sprung contacts to the card. Also mentioned by others are SLC industrial grade cards which have more stable flash memory technology, better controllers, wider operating temperatures and not to mention are much much faster. It all comes down to cost, but trying to build a reliable system with removable cards is not a good idea. The Pi design team should consider adding some reasonable quality onboard eMMC memory in the future. Regarding cards, I can vouch for http://www.atpinc.com http://www.atpinc.com which we use where I work, expensive but very reliable.