You open your disk Management Tool and see your Dynamic disk as Healthy but it says (At Risk) on them.
So what is really happening.

In general this kind of problem happens when you have any I/O error, maybe a latency in the SAN, maybe one disk write or read fails. So be aware and check if everything is ok.

Clicking in Re-activate will clean this error, but if it keeps appearing take care, something very bad is happening and can corrupt your data.