I just got an email from Other World Computing which stated:
"You want a drive at least as large or larger than the drive you are going to be using the Time Machine drive for." In my case, I think that's overkill, right? My Macbook holds 500 GB. I didn't quite understand all that you said, but as long as I get an external drive of at least 150 GB, I think I will be safe from what I've deduced. Also, if I get crazy with it, I can always buy a larger capacity hard drive.
Rita
To estimate the space needed on a Time Machine drive you could start with the basic OS install you start out with on your Mac, and add to that the data you move to it now plus expect to add later. Since the TM allows you to store multiple versions of whatever you have on your Mac, you'd have to accommodate that too, at least to some extent. You should also know that when a TM drive gets full, TM will start deleting the oldest data to make room for the new.
In summary, the size of a TM hard disk would amount to the used space on your Mac HD you begin with, plus a (judicious) multiple of the space your data takes up (or likely will take up). So, if for example the OS measures 10GB and your data too, you'd need at least 20 GB, but probably more like 40-60GB. As it's getting hard to find HDs smaller than 150GB or so (excluding SSDs), you could start with one of those, and shop around for reliability, price and perhaps variables like multiple port options. As it happens, the price range of such drives start at around $90 (like Ira's choice), so that seems like the bottom line unless you opt for a raw drive mechanism plus dock.
