I've heard that due to bugs all high bandwidth devices are flakey.
They are getting better and better though :-)
Specifically, MMC and SATA media are unreliable and the best solution is to try USB.
With the latest official firmware (and IO FPGA) from the VE CD 3.0:
https://silver.arm.com/browse/VE030
MMC with the default card clock of ~500kHz is rock-solid :-) Personally I actually use it with mmci.fmax=12000000, which makes it rather fast. The downside is that I can't use any USB devices at the same time (well, you can do some tricks with IRQ core affinity to make it work). So 6000000 (6MHz) is a safe compromise.
I have just copied my 11.08 image to MMC and tried booting off that. I found that it does seem to hang but after 5 minutes or so it eventually gets to the command prompt. Once booted, using ssh to copy a large file gave a transfer rate of about 30kB/s.
With the increased MMC card clock it becomes really usable, but then you may face an issue with the Ethernet ;-)
https://bugs.launchpad.net/linux-linaro/+bug/673820
Which can be fixed with even newer IO FPGA files, as mentioned in the bug log.
Cheers!
Paweł