SQL Server FCI online with incorrect file path I just encountered this strange issue with my SQL FCI yesterday.

Windows 2008 R2 and SQL 2012 - 2 nodes FCI

SQL server/ SQL agent resources are online and working as normal, but the system databases are now on a different drive. 

They are now in drive X as opposed to drive K and drive K is empty. sp_helpfile and SQL startup parameter are showing the old drive path which is drive K. System db backup jobs failed because they were still pointing to the old path, which is expected. Now drive X has all the files that K used to have and K now is empty and has a new size.

I know SQL won't start if the file path is different because the master database files are not there. However, we never restarted SQL service and somehow the system database files were moved to the new drive without interrupting the SQL resources. 

Has anyone encountered this behavior? My guess would be X is formerly K and the files didn't get moved, but how could SQL could still be on pointing to the old drive path K?Disk X is also under SQL server dependency. 

Thanks.
Yes, it could happen if you add a new disk to the cluster. The new disk could use that same drive letter and force the another cluster disk to use the new letter (which happens to be the disk that holds the system databases). Since the dependency remains the same, changing the disk letter doesn't affect the SQL resources. As a result, SQL services would still be up but the startup parameters would still pointing to the old drive.