The answer is probably obvious, but I'm sure enquiring minds want to know: if I erase a disk and reformat it, what's already taking up almost half a gigabyte of space? In this case, it's formatted HFS+ journaled on Mac OS X.
You don't say what make and model disk, how you arrived at it's expected capacity or how you measured actual available capacity - so it could be some or all (or none) of the following:
450 MB is a lot to explain away on a 1 GB disk, but not so problematic on a 2 TB disk.
There's a discussion on macworld that says
Apple have published some details of HFS+ I'm pretty sure that if you start to read this, before you reach the end you'll stop worrying about that "missing" 450 MB.
I liked the bit where it discussed a "Metadata Zone" which is only used for metadata (obviously) and "hot files". Obviously less space for all your boring files. Presumably OSX tools that report "free disk space" include free space in the Metadata Zone in the total reported free space?
You can use
Using DiskWarrior (a fantastic app, btw), after clicking to Rebuild that volume and looking in Console.app, I could see it report the following:
DiskWarrior App: disk4s6 Extents, Catalog and Attributes tree sizes total 139 MB.
So it appears that the "missing space" consists primarily of the disk directory itself (these are the "files" shown in the image above in RedGrittyBrick's post).