My book studio for mac

trying to install external drive on mac.the screen asking if i want to use for time machine back up does not come skips this every time.using a different hard drive for back up

Hi there, what if you manually open Time Machine? o.O

Click on Go > Applications > Time Machine.

Remember that your Mac needs to be at least 10.5 to have the tool. 

You can manually select a backup disk in Time Machine System Preferences if you want to.

  1. Choose From the  Apple  menu, choose  System Preferences.
  2. From the  View  menu, choose  Time Machine.
  3. Click “Select Backup Disk”.
  4. Choose a disk where backups will be stored, then click “Use for Backup.”