On error goto NoDrive
Dir( "x:\" )
...

exit ..
nodrive:

'не забудь   Declare Function GetLogicalDrives Lib "kernel32" () As Long
Dim TestDrive As String
Dim NumDrive As Long
Dim LngBytes As Long
TestDrive = Me![P_1] ' имя драйва '
NumDrive = Asc(Left(UCase(TestDrive), 1)) - 65
LngBytes = GetLogicalDrives()
If (LngBytes And (2 ^ NumDrive)) > 0 Then
                                 MsgBox ("Yes " & TestDrive)
                               Else
                                 MsgBox ("No " & TestDrive)
End If
'GetDriveType - вернёт тип дисков