'this sub splits the file
Sub Split()

Dim tFile As String
Dim tPath As String
Dim tSplitFile As String
Dim tChunk As Single
Dim Txt As String
Dim n As Long, nLength As Long
Dim m As Long
Dim Finished As Boolean

'get user input
tFile = Range("Filename")
tChunk = Range("Chunk")

'check source file exists
On Error Resume Next
If Dir(tFile) = "" Then
  MsgBox "I can't find the file. Please specify the full path and name", vbExclamation
  Exit Sub
End If
On Error GoTo 0

'initialise
tPath = Left$(tFile, InStrRev(tFile, "\"))
nLength = FileLen(tFile) '+ 1

'delete any existing split files
n = 0
Do
  n = n + 1
  tSplitFile = tPath & "Split" & n & ".dat"
  If Dir(tSplitFile) <> "" Then Kill tSplitFile Else Exit Do
Loop

'do the split
n = 0
Open tFile For Binary As #1
Do
  'the last chunk will be what's left over...
  m = nLength - n * tChunk
  If m > tChunk Then
    m = tChunk
  Else 'last chunk, mark as finished
    Finished = True
  End If
  'set up dummy string of correct length & read in data
  Txt = String(m, " ")
  Get #1, , Txt
  'write to file
  n = n + 1
  tSplitFile = tPath & "Split" & n & ".dat"
  Open tSplitFile For Binary As #2
  Put #2, , Txt
  Close #2
Loop While Not Finished
Close #1

'test result - True parameter tells Excel this is a test only
If CombineSplits(True) Then MsgBox "The file has been split successfully"

End Sub

'combines files - triggered by user click
Sub Combine()

CombineSplits

End Sub

'combines files
'Testing parameter tells Excel if this is a test only
Function CombineSplits(Optional Testing As Boolean = False) As Boolean

Dim tFile As String
Dim tFile0 As String
Dim tPath As String
Dim tChunk As Single
Dim Txt As String
Dim n As Long, nLength As Long
Dim m As Long

'if testing, create a dummy filename to put the results into
If Testing Then
  tFile0 = Range("Filename") 'actual original filename
  n = InStr(tFile0, ".") - 1
  tFile = Left$(tFile0, n) & "_a" & Mid$(tFile0, n + 1) 'dummy filename
  tChunk = Range("Chunk")
Else 'we're combining for real, watch out if file exists already
  tFile = Range("Filename")
  If Dir(tFile) <> "" Then
    If MsgBox("The filename you have specified exists already. Do you want to overwrite it", vbQuestion + vbYesNo) <> vbYes Then
      Exit Function
    End If
  End If
End If

tPath = Left$(tFile, InStrRev(tFile, "\"))

'do the combining
n = 0
Open tFile For Binary As #1
Do
  n = n + 1
  Txt = Dir(tPath & "Split" & n & ".dat")
  If Txt = "" Then Exit Do
  Txt = String(FileLen(tPath & "Split" & n & ".dat"), " ")
  Open tPath & "Split" & n & ".dat" For Binary As #2
  Get #2, , Txt
  Close #2
  Put #1, , Txt
Loop
Close #1
If n = 1 Then
  MsgBox "I didn't find any files to combine", vbExclamation
  Exit Function
End If

'if testing, compare dummy result with original, bye by byte
If Testing Then
  Dim b1() As Byte, b2() As Byte
  FilesBinaryRead tFile0, b1()
  FilesBinaryRead tFile, b2()
  'kill dummy file
  Kill tFile
  
  For n = 1 To UBound(b1)
    If b1(n) <> b2(n) Then
      MsgBox "The split failed at byte " & n
      Exit Function
    End If
  Next n
  CombineSplits = True
Else
  MsgBox "The files have been recombined"
  CombineSplits = True
End If

End Function

Sub FilesBinaryRead(ByVal sFileName As String, ByRef bb() As Byte)
    Dim ihwndFile As Integer
    
    'Open file
    ihwndFile = FreeFile
    Open sFileName For Binary Access Read As #ihwndFile
    
    'Size the array to hold the file contents
    ReDim bb(1 To LOF(ihwndFile))
    Get #ihwndFile, , bb
    Close #ihwndFile
    
End Sub