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Проблема вроде как решилась после замены проверка на wpInstalling вместо wpReady »

Интересное высказывание, главное уверенное... надо так:

Пример

Код:

[Setup]
AppName=My Program
AppVerName=My Program v 1.5
DefaultDirName={pf}\My Program
DefaultGroupName=My Program
OutputDir=.
Compression=lzma2/ultra
InternalCompressLevel=ultra
SolidCompression=yes

[Languages]
Name: ru; MessagesFile: compiler:Languages\Russian.isl
Name: en; MessagesFile: compiler:Languages\English.isl

[Types]
Name: full; Description: Full installation
Name: custom; Description: Custom installation; Flags: iscustom

[Components]
Name: comp1; Description: Component 1; Types: full custom
Name: comp2; Description: Component 2; Types: full

#define ITDRoot ReadReg(HKEY_LOCAL_MACHINE,'Software\Sherlock Software\InnoTools\Downloader','InstallPath','')

[Files]
Source: {#ITDRoot}\itdownload.dll; Flags: dontcopy nocompression solidbreak
Source: {#ITDRoot}\languages\*.ini; Flags: dontcopy nocompression solidbreak

[Code]
#include ITDRoot + '\it_download.iss'
procedure LoadITDLang;
var
  lang: string;
begin
  { заранее нужно перевести на русский файл *.ini }
  lang := ExpandConstant('{language}');
  try
    ExtractTemporaryFile('itd_' + lang + '.ini');
    ITD_LoadStrings(ExpandConstant('{tmp}\itd_' + lang + '.ini'));
  except
  end;
end;

procedure InitializeWizard();
begin
  ITD_Init; // Currently a NOP. Don't count on it in future.
  LoadITDLang;
  ITD_DownloadAfter(wpReady);
end;

function NextButtonClick(CurPageID: Integer): Boolean;
begin
  Result := True;
  if CurPageID = wpSelectComponents then
    begin
      ITD_ClearFiles;
      if IsComponentSelected('comp1') then
        ITD_AddFile('http://www.sherlocksoftware.org/petz/files/dogz5.zip', ExpandConstant('{tmp}\dogz5.zip'));
      if IsComponentSelected('comp2') then
        ITD_AddFile('http://www.sherlocksoftware.org/petz/files/petz4.zip', ExpandConstant('{tmp}\petz4.zip'));
    end;
end;

Как разделить установку на 2 диска »

Почитайте в Справке, секция [Setup], разделы DiskSpanning и DiskSliceSize.