You might see an  ASSERT.TXT appear on the MICROBIT drive if there has been a problem with flashing a program over USB. It might appear on its own, or alongside a FAIL.TXT

The ASSERT.TXT file details where an assertion failure occurred; providing the source file and line number. 

File: ..\..\..\source\daplink\drag-n-drop\flash_manager.c 
Line: 131 
Source: Application

The assertion failure can be cleared by deletingASSERT.TXTfrom the drive, which will cause the MICROBIT drive to remount.