I finally convinced myself that my new eSATA drivers were working, so I switched my two main external drives back from USB to eSATA. Everything seemed hunky dory, until I noticed that Time Machine was suddenly spinning for an awfully long time. I checked the logs and saw that Time Machine was busily deleting all my old backups. By the time I noticed, I had already lost about a year's worth.
What happened, as it turned out, was that when I unmounted the external drives to switch them back over to the eSATA cables, Time Machine removed those volumes from the exclusion list. It was trying to backup those external drives. To do that it needed about a terabyte of free space, so it was busily deleting all my old backups to make room. If I hadn't stopped it, it would have nuked them all. My backup volume is only 750GB.
IMHO this is a SERIOUS bug in Time Machine, almost bordering on legally actionable negligence on Apple's part. There are apparently people out there who have lost all of their backups because the exact same thing happened to them but they didn't notice in time. Newly mounted external drives should be excluded from backup by default. At the very least, Time Machine should prompt you, or warn you, or something. What any backup program should NOT do under any circumstances (and I would have hoped this would go without saying, but apparently not) is silently delete all of your backups.
UPDATE: There seems to be some confusion on two points. First, there was an option in TM on Leopard to "warn before old backups are deleted." In SL that option has been changed to "notify after old backups are deleted", which seems to me to rather defeat the purpose. Second, in most jurisdictions, if you accept money for a product there are implied warranties of merchantability and fitness for a particular purpose which you cannot disclaim. In this case, TM is advertised as an integral feature of OS X whose purpose is to make backups of your data. If instead it deletes all your data, that *could* be negligence that Apple could not legally disclaim, rather like selling a fire extinguisher that actually set your house on fire. But IANAL, and I didn't suffer any actual damages because I caught it in time, so I will not be putting this theory to the test. But someone who lost all their backups might.