Hi Pete,
Your TM partition is 1.7TB and after your initial backup you have 1.58TB free. This means that your initial backup of your entire boot drive was roughly 120GB. Your TM partition is roughly 8 times this size. Now I don't know what type of work you do and thus how fast you will be generating new/modified files, but given that your boot drive was only 120GB then it is safe to assume that you are not working on very large data sets and consequently your rate of generating modified files that take up large amounts of new TM backups will be fairly small. Thus a TM partition that is roughly 9 times the size of your boot drive used space will most likely last you years before anything is deleted form your TM backups. In my opinion, you do NOT need a new drive for your TM partition!
...just my two cents worth of free advice, and, as always, you get what you pay for...
Regards,
Switon
P.S. If you look at my previous posting entitled "RE: TM Backups...", you will see that you haven't lost anything from those 2-6 day old backups being automatically deleted by TM (TM only stores the last 24 hours of the hourly backups). You can return to any of the daily TM backups and recover what was on your boot drive that day (for the last month of daily backups).