Question: 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.
Answer: 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:
Disk vendors state sizes in powers of 1000,Operating systems report sizes in powers of 1024 (recent OSX doesn’t)
Filesystems reserve space for Filesystem structures like inode tables.Journalled filesystems reserve space for Journals.
Some disks have special partitions on them, e.g. for recovery (apparently OSX doesn’t)
low-level formatting involves writing data to the disk to mark out sectors, this uses some of the notional capacity of the disk. (or maybe not)
Disks can contain bad sectors that the drive electronics mark as such.(perhaps not an issue)
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
man newfs_hfs
-J [journal-size]Creates a journaled HFS+ volume. The default journal size is8MB. Appending an ‘M’ to the journal size implies megabytes(i.e. 64M is 64 megabytes). The maximum journal size is 512megabytes.
and
HFS Plus uses a number of interrelated structures to manage the organization of data on the volume. These structures include:
the volume headerthe catalog filethe extents overflow filethe attributes filethe allocation file (bitmap)the startup file
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?