The difficulty you are experiencing is caused by an incompatibility between the Startup Disk control panel version 7.5.2, which is part of System 7.5 Update 2.0, and Quadra and Centris 660AV and 840AV computers.
This control panel writes information to the parameter RAM (PRAM) to identify the hard disk from which the computer will start up. In this case, the information is not being stored properly.
Because the control panel setting is not working, the computer will simply use the SCSI device with the highest SCSI ID to start up. An external device will typically have a higher SCSI ID than your internal hard disk (the internal hard disk that shipped with your Macintosh computer has a SCSI ID of 0).
There are currently two ways to work around this. First, you can use an older version of the Startup Disk control panel. Startup Disk control panel version 7.3.2, which comes with System 7.5 or System 7.5.1, will work without complications.
Secondly, you can reset the PRAM to reset the default startup disk to your internal hard disk. To reset the PRAM, restart the computer and hold down the Command (Apple), Option, P, and R keys. Wait for the computer to make its startup chime twice. You may then release the keys. Be aware that resetting the PRAM will reset such settings as the sound volume and mouse speed to their default value, and will purge the contents of a RAM disk if you are using one.