found this on here a few months back cant remember who originally posted. Is this what you are looking for?
- (uint64_t)freeDiskspace
{
uint64_t totalSpace = 0;
uint64_t totalFreeSpace = 0;
__autoreleasing NSError *error = nil;  
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);  
NSDictionary *dictionary = [[NSFileManager defaultManager] attributesOfFileSystemForPath:[paths lastObject] error: &error];  
if (dictionary) {  
    NSNumber *fileSystemSizeInBytes = [dictionary objectForKey: NSFileSystemSize];  
    NSNumber *freeFileSystemSizeInBytes = [dictionary objectForKey:NSFileSystemFreeSize];
    totalSpace = [fileSystemSizeInBytes unsignedLongLongValue];
    totalFreeSpace = [freeFileSystemSizeInBytes unsignedLongLongValue];
    NSLog(@"Memory Capacity of %llu MiB with %llu MiB Free memory available.", ((totalSpace/1024ll)/1024ll), ((totalFreeSpace/1024ll)/1024ll));
   } else {  
    NSLog(@"Error Obtaining System Memory Info: Domain = %@, Code = %@", [error domain], [error code]);  
}  
return totalFreeSpace;
}