Advice Needed on Drive Failure

I have a 2007 Series 3 TiVo and I added a 500GB WD DVR Expander when it became available (late 2008, if I recall). I have Digital Cable, with two cable cards and a Tuning Adapter.

I would get infrequent Tuning Adapter Reconnect errors over the years, but starting this July, they became more frequent, at least daily, and up to 3 or 4 times while watching a half hour show. I removed the Tuning Adapter in order to replace it, and over a few days without the Adapter I observed the TiVo unpredictable stuttering and freezing. I figured I was seeing the beginning of a hard drive failure, and it probably wasn't the Tuning Adapter's fault.

Audibly, the TiVo seemed to be making some noise, maybe just the internal fan, and the DVR Expander was very quiet, so I've been anticipating that the internal drive was failing. Meanwhile, newly recorded shows were having stuttering problems, but older shows are still perfect - no stuttering or freezing at all. I figure, I'll watch shows that I can't move (HBO's Game of Thrones, for instance) and then deal with replacing what hardware I need to. I've noticed that shows that I'm permitted to copy, but probably contain some stutters will not completely copy via TiVo Desktop.

This morning I first ran the Kickstart 54 program, and both drives passed the Initial State, but to my surprise the SMART test thus far shows the secondary drive Fail 7 on the Short Test, the Conveyence Test and the Extended Test. The Primary Drive has thus far passed Short and Conveyance, and I'm plowing through the Extended test with it. [The Off-line scan for the Secondary indicates it will take 233 minutes! I don't know if I've got anything left to prove.]

I figure I should probably repeat the SMART tests, wrap up watching what I need to watch (maybe another week), and then disconnect the DVR Expander, and make sure the Series 3 will run standalone without errors. If so, I will probably get a new 1TB DVR Expander ($99) rather than beef up the internal drive - 1TB replacement costs $249, although given that price a new Premiere starts to make more sense.

If any one sees some flaws with this plan, or has some other ideas and tests to suggest, I'd appreciate hearing them.