Sorry, my checkout was out of date, and from before the merge of the 10.9 pull request.
This merge includes a call to sync_volume_np (in MemoryCardDriverThreaded_MacOSX::Unmount) which was not added until 10.8, so we basically killed pre-10.8 compatibility with this change.
int sync_volume_np(const char *, int) __OSX_AVAILABLE_STARTING(__MAC_10_8, __IPHONE_NA);
Restoring the old version of Unmount allows the current master branch to build with the 10.6 SDK.
There's a #define that can be checked for this, right? It'd be nice to still allow building against the older SDK.
(also, I should really update the DL page! It's on my todo list...)