If you use WinMFS to mfscopy your 750G drive, you can upgrade to a 1TB drive while keeping your recordings. Adding an external drive will also allow you to keep the recording that you already have on your machine. Since you have already upgraded your internal drive, you will need to marry the drives using WinMFS (I believe, I know this is the case for TivoHDs).
You can also upgrade your internal with a 1TB using mfscopy and THEN add an external drive (up to another 1TB), while still keeping your recordings.
1.5TB drive are not currently supported by tivos. Max single drive is 1TB.
Tivo Premiere 2TB WD20EARS (lifetime)
TivoHD 1TB WD10EACS
+ 1TB WD10EVCS Antec MX-1 (lifetime)
TivoHD 1TB WD10EACS (lifetime)
Series2 540 400GB Seagate DB35 (lifetime)