False-failures when mirrored in high-safety mode without a witness
OK, so this is high safety without automatic failover with no witness.

Here's the official books online answer:


  If the mirror server instance goes down, the principal server
  instance is unaffected and runs exposed (that is without mirroring the
  data)." Source


I have seen that to be true in my experience. In other words, disconnected, but available for reads and writes. The transaction log will grow until the situation is fixed, because it needs to keep the info to send to the mirror when it comes back around.

Note: I absolutely recommend setting up a test environment for yourself before you make any changes. Don't just take my word for it. You want a place to be able to stage the changes and need to test what happens for yourself before going live.

Edit: As for what happens when the mirror comes back, typically it will automatically resume in my experience, but I know of cases where it hasn't and TSQL had to be run to make it resume. Very easy to simulate in a demo environment to get yourself comfortable with it.