I don't think there is any difference between "unallocated" and "free" in this context - but there are a number of ways such drive space can be unusable. The most common is to not have the free space actually inside the extended partition.
If you run gparted it will show you what the situation is, or
and figure it out from the partition sizes.
If you don't have a working linux, you can download the gparted Live Disk, which is a handy tool to have for emergencies.