>How do I determine if drive A: has a disk in it>
>
>I tried this code, but the error handler fails
>to trap the error
>
>ON ERROR MESSAGEBOX(ERROR())
>ADIR(a, "a:")
>ON ERROR

The easiest way to get around the error is to disable the OS from intercepting it. Try:

DECLARE INTEGER SetErrorMode IN WIN32API INTEGER ErrorMode
#define SEM_NOOPENFILEERRORBOX      0x8000
#define SEM_FAILCRITICALERRORS      0x0001
nErrorModeOld = SetErrorMode(SEM_NOOPENFILEERRORBOX + SEM_FAILCRITICALERRORS)
IF DISKSPACE('A:') <= 0
   ? 'No Disk'
   * No A: disk
ENDIF
=SetErrorMode(nErrorModeOld)

A better way is to use the WSH FileSystemObject's IsReady method:

oFSO = CREATEOBJ('Scripting.FileSystemObject')
oDrive = oFSO.GetDrive('A:')
? oDrive.IsReady()