Time Machine works, but when disconnected keeps saying "Could not complete backup"

Running Snow Leopard 10.6.8 on a late '08 MBP: While connected to my work's external 1TB drive, TM operates as expected. Every day I disconnect and take it home. Without the TM drive available, every so often I get "TM could not complete the backup."

Suspecting a glitchy .plist, I moved com.apple.timemachine to my desktop and hit "Back Up Now." Evidently now the new plist doesn't recognize the >600GB of TM data logged, as, of course, it shows no previous backups (image attached). I feared that proceeding would mean it would begin an entirely NEW routine—rendering my existing TM useless. So I stopped, replaced the previous plist, and (whew) again TM shows my history.

So it's not a critical issue, but a regular annoyance: why does TM now think it has to tell me every time it didn't work? This started up maybe a week or two ago, and I don't recall any system modifications. I see nothing in the TM prefs that would trigger this.

Any ideas? Thanks.