попробуйте
NSError *error = nil;
path = [path stringByAppendingPathComponent:@"tempfile"];
NSURL *URL = [NSURL fileURLWithPath:path];
[[NSFileManager defaultManager]createFileAtPath:path contents:nil attributes:nil]
NSFileHandle *writeFile = [NSFileHandle fileHandleForWritingToURL:URL error:&error];
[writeFile truncateFileAtOffset:4000000000*sizeof(Byte)];