Accessing a hard drive larger than 137 Gig requires LBA-48 addressing. The kernel in early Series 2 TiVo and DirecTiVo software did not support LBA-48 addressing; the kernels supplied in later versions did. Off the top of my head, the kernels in 6.2 (DirecTiVo) and 7.1? (TiVo) and later software did.
Both statements are true in context, but apply to different hardware. Series 2 TiVos and non-HD DirecTiVos could be exploited using the monte technique (loading an early exploitable kernel, then transferring control to the latest kernel that had been 'neutered' with killinitrd). The Series 2 HD DirecTiVo firmware blocked that exploit, but contained a flaw that allowed access by modifying the kernel with killhdinitrd. That exploit also worked for earlier TiVos and DirecTiVos, which simplified hacking. With the Series 2.5 TiVos (Nightlight) and DirecTiVos (R10) that firmware flaw was eliminated. The TCD649080 is a Series 3 and as far as I know the only way to hack them is by replacing the prom (firmware). At present Series 4 TiVos cannot be hacked so far as I know.
That is above my experience level, but as far as I know, you don't. HOWEVER telnet / ftp is vital for maintaining proper operation of a hacked TiVo. TiVo does a good job of keeping their software up to date, but when you 'enhance' their operating system YOU have to take care of some of that.
Whoo!!! That's quite a list, and it raises more questions, and contains more difficulties.
1. Your TCD649080 already contains the software. No need to find an image unless yours is damaged. Burning it to CD is ALWAYS a good idea.
2. I don't recommend Hinsdale / mfs tools for this. Rather, use MFS Live (Google it), it contains additional features.
3. Strictly speaking, I don't believe you need to do anything with the initrd to prevent software upgrades. Adding upgradesw=false to the boot parameters should do this, and I don't believe it is checked by the firmware or the OS.
4. HOWEVER, that opens another can of worms - running a TiVo with a down-level OS is a bad idea. Eventually you will be forced to update to a newer OS just to get guide data.
PlainBill