As anyone who has looked into upgrading their Windows Phone 7 device’s MicroSD card has discovered, Microsoft did not intend for these cards to be removed or swapped out until WP7-certified cards have been released. Those who tried anyway ran into a range of issues such as rebooting, data loss, and poor performance.
wm.andrew.wilson writes about a registry hack that causes the SD driver to enable power management features. Apparently these features being disabled are the major cause of the issues experienced, and they go away entirely once the change is applied, along with a performance boost. The relevant key to change in this case is:
HKEY_LOCAL_MACHINEDriversSDCARDClientDriversClassSDMemory_ClassDisablePowerManagement (set to 0)
It is notable that these same power management keys have been present in the Windows Mobile/Windows Compact Edition SD/MMC card driver for at least five years; these same registry changes were recommended to increase battery life on many older WM devices. Looking closely at Microsoft’s platform builder (the tool used to build WM/CE/WP7 firmware images), these power management keys are by default un-set. Are OEMs disabling these power management features and inadvertently causing problems?