If when creating a Storage Spaces volume you used a "thin provisioning" feature, then sooner or later you face the situation when physical space for your virtual hard drive is over. Then, if you need to write data, the Storage Spaces driver can no longer allocate a new slab; it just disables the virtual drive, i.e. makes the drive offline, interrupting the last write operation. To resolve this problem you need to add disk space to the pool. However, it is not always possible, for example in case of four-disk array you would have to bring four disks at once. So if for some reason you cannot add disks immediately but you need to get access to the data, you can go to Windows Disk Management, make the disk online again, then delete unnecessary files and folders. For a while this should be enough but you will have to add disks eventually.
However, this doesn't always work. It may happen that whenever Windows Disk Management brings the disk online, the Storage Spaces driver immediately makes it offline again and you just do not have time to delete files to free disk space. The solution is to create several virtual hard drives (.vhd) of small sizes (say, five GB per disk but not less than four GB each) and add them to the pool.