Yep, you should not erase or use this System Reserved Partition, as it is used for GPT and/or (U)EFI -- machine internals, don't bother about them (or Wikipedia them).
Being a quite small partition, I believe you will not suffer by losing this disk space.
Now, back on your original question, yes, your missing disk is that "Unallocated space" as mentioned above by Bill2. Just create a file system on it, assign a drive letter and you should be ready. Also worth a mention: In case you had any data on that disk, I'm afraid it may now be gone. There is no file system, thus there are no files.
Hope you was installing a clean machine...