What is the SQL Server cost, in terms of performance, of sharing physical disks? I am an application user in Health Care.  The application relies on communication with a large database, including image data and other large numeric arrays.  We add approximately 15GB per day to the database server (note image data might be file based).  The application crashes, sometimes coincident in time with querying the database e.g. when you query the patient list in opening a new case. 

There are two instances of the application database, used by two groups, that currently share a physical disk in the database server.  I wonder what factors I'd need to understand in order to estimate whether or not there might be appreciable performance gain by giving each instance of the database a dedicated physical disk.
A performance factor which we saw in one of our environment was related to  high disk read write activity from one database on those shared disks. Since one had a higher transaction activity using the storage hitting hard caused slownesss for other. Also it becomes hard to troubleshoot when you have shared instance for those disks. We moved them to their respective virtual servers instead.

Also other factors which made us move was difficulty in performing any reboot activity after patching as one lob would be fine while other conflicting.

Moreover backup of databases in one instance at x timings may impact performance of other sql instance depending upon how big the servers we are talking and how good or bad the storage is.