To me it looks like
Simple Volume is something "modern", inaccessible by Windows XP Home. When googling I've found some problem with it, e.g., here, here, and here, but no advantages. One of my disks is a "simple" volume as well, and I really don't know why.
Using Linux I've found out that the disk has an invalid partition table, would simply overwriting it help (I know I'd lose all the data on it)?
I'm using Windows XP 64 Professional and can use the disk, just can't convert it to a normal one using my Windows. I can't extend the volume (it's grayed out), probably because it already spans the whole disk. I can't shrink it either, there's nothing like this.
I wonder what's the purpose of Simple Volumes, and why they (unlike RAID/LVM volumes in Linux) break partitioning.
Just as the disk had automagically become "Simple Volume" some time ago, today it decided it was no good idea and converted itself back to a normal disk, throwing all the data away. Fortunately, it was right in that the data were quite useless anyway (some old garbage to be sorted out). I tried to convert it to "Dynamic disk" again, but "The operation didn't complete. Check the System Event Log for more information on the error." And there's nothing about it in the Log. Sure, it may be a HW problem, but it looks rather as a bad joke.