My self built PC has to SATA connections. On the first,I have an aging XP PRO SP2 system and on my second I am developing a nw XP Pro SP3 system. When working on each, the power to the other is disconnected.
When I installed Windows and SP3, and then put Partition Magic onto my F:\ drive, I found that I had used about 4.8 GB of my 7.6 GB C:\ drive. I then installed Norton to my F:\ drive (very little gets ontot the C:\ drive, and my HP printer ro my C:\ drive. I was amazed to realise I only had about 400 Mb of space left on my C:\ drive, but on furyjer investigation - with showing all hidden and system files selected and with Unhide protected and OS files unchecked - that all I had on the C:\ drive was:
Documents & Settings folder - 269 MB HP Folder 1.06 GMB Program Files Folder 921 MB Recycler Folder 7.87 kb System Volume Inf Folder 0 Windows XP Folder 2.48 Gb Page File 1.47 GB Hiberfil 0.98 Gb Sundry small files 293 Gb
Total - 6.1 Gb
So I seem to have lost about 1Gb of space. As it happens, I can go up to 15Gb but I still wonder where I've lost an entire Gb of space.
Some of it will be slack space. That's the unused space in a storage cluster where a file doesn't fill the cluster.
It's pretty difficult to calculate, and tends to increase with the number of files, rather than the size of files - as each file (or file fragment if the file is non-contiguous) can only have one piece of slack space.
There is always a discrepancy between the total amount of data (sum of all files sizes) and disk space used (sum of all allocated blocks.)
This note by MS also explains how and where disk space may be used by NTFS file systems in other ways. click here