Michelle,
It appears that the product team removed the Disk agent in 5.0.
As for the official reasons, I am not certain, but I can tell you that in the 10 years I've been doing Storage Fondation and VCS, I've used the Disk resource maybe once.
Generally speaking, with regards to storage resources in a service group, I tend to be a bit of a minimalist.
What I tell students and customers is that if you have a filesystem on the volume, your resource depencency tree should be Mount -> Diskgroup
Some people say that you should go Mount -> Volume -> Diskgroup, but frankly, I only use the Volume resource when I am dealing with Raw Volumes. The Diskgroup agent can auto-start volumes in the diskgroup, which most times is what I want. Then the Volume resource becomes completely surperfluous.
If the volume resource goes south... most times it's going to be a diskgroup failure, and the diskgroup resource will fault. In the unlikely event that the volume croaks, but the diskgroup remains alive, the Mount resource will be the one to fault. Because of that, my opinion is that you don't need the Volume resource in the middle.
If you don't have a filesystem (i.e. Mount resource) on the volume because it's raw, then you really DO need something that will check the health of the volume, in case the plexes become unusable, but the diskgroup doesn't fault.
As for the Disk resource, you have SF 5.0 on your system, so you shouldn't need the Disk resource.
I believe I remember someone saying to me not too long ago, that if the Disk resources fault, the mount point or volumes will fault too, so for much the same reason as above, the decision was to remove the Disk resource from the bundled agent list on Solaris. There may be other, more pertinent reasons, but the bottom line is, you don't really need it.
