Date   : Sun, 14 Nov 1982008:56:00-EST
From   : X.GYRO@Mit-Mc
Subject: when to log-in a new disk?

There is a well-defined (though perhaps not documented; I forget) way
for the BIOS to know about BDOS disk resets.  When the BDOS calls
SELDSK, bit 0 of E is 0 if the BDOS thinks the disk is being accessed
the first time, else 1.

-- Scott