This happens when a Synology loses power. vCenter does not automatically reconnect the datastore. 

1. remove the disk from the server that is affected. 
   1. right click the server and select Edit Settings
      
   2. Expand Hard Disk 2 and take note of the Disk File Name. 
      
   3. click on the three dots next to the affected disk and select Remove Device. 
      
2. go to the datastore in vCenter and right click on the affected datastore. Take note of the Datastore Name. It will be needed later.
   
3. Click on Unmount Datastore and select the datastore to unmount and click OK
   
4. Log in to the Synology and go to Shared Folder in the Control Panel
   
5. Select the shared folder and click Edit and select NFS Permissions
   
   
6. Take note of the Mount Path listed at the bottom of this screen
7. Go back to vCenter and right click on the server. 
   
   1. Select Storage>New Datastore and select NFS
      
   2. Click Next and select NFS 3. Should already be selected
      
   3. Enter the datastore information
      
      1. The Name is the name you took note of when unmounting the datastore from step 2 above
      2. Folder is the NFS path you took note of in step 5 above
      3. Server is the IP address of the Synology
   4. Click Next
   5. Select the Host and click Next
      
   6. Verify the information and click Finish
      
   7. The datastore should now be added to vCenter
      
8. Go to the server that the storage was removed, rigkt click the server and select Edit Settings
   
   1. Click on Add New Device and select Existing Hard Disk
      
   2. Select the Datastore that was just created.
      
      
   3. Select the folder and the file that represents the storage. This is the Disk FIle Name you took note of in step 1.2
      
   4. The new hard disk will be listed. No changes are necessary. Select OK to add the disk.
      
9. Power on the server.