What memory card do you recommend for the new Canon 70D? Just wondering if I need to upgrade from a San Disk 30GB Class 10 (45MB/sec).
A seemingly simple question, most likely expecting a relatively simple answer, but the details get messy.
The Class 10 rating of your current cards means that the card, in a non-fragmented state, is capable of sequential reads and writes at a minimum of 10MBs and applies to the Normal and High bus speeds. A card that also supports the Ultra-High Speed I (UHS-I) bus (speeds 100MHz and 208MHz) also has its own class designation -- the "U" with the 1 inside also indicates a 10MBs minimum. These speed class ratings are defined in the SD standards and stored in the card so the host devices (eg cameras and card readers/computers) can read them and proceed accordingly.
Maximum speeds are left to the manufacturers to claim (what could go wrong there?) and sometimes reflect read speeds, sometimes write speeds, occasionally both. In any case you need to dig somewhat to find out which. Manufacturers could really be more forthcoming and consistent.
One more item if your largest current card is SDHC. SDHC maxes out at 32GB. For larger than 32GB up to 2TB, you are looking at SDXC cards, which have the same speed classes and bus speeds as SDHC, so all that still applies (but not true when you get to UHS-II which haven't hit the market yet), but instead of being formatted FAT32, they use the exFAT file system (developed and patented by Microsoft). If you are on an up to date Windows 8, 7, Vista SP1+, or XP with KB955704 applied, you are ready to go. Even the latest Macs support exFAT (starting with OS X 10.6.5?). The problem is Linux and the Microsoft patents. If you are using Linux (or other *nix) exFAT is not supported. Therefore with Linux you can either not go larger than SDHC's 32GB or you can re-format your SDXC with FAT32 (which could potentially introduce problems to the card and / or host device as it does violate the standard).
At least for the 70D, Canon has said it supports SD, SDHC, and SDXC as well as UHS-I. (Of course more detailed info on what speeds it can support within those standards would be appreciated.)
I would recommend SDHC UHS-I and SDXC UHS-I cards (with the SDXC/exFAT caveat above). The number of GB depends on your shooting preferences and budget. Claimed max speeds likewise -- more important if you often shoot high speed bursts for sports or BIF, not so much for landscapes.