Using SD with Time Machine

[Estuary]

I backup to an EHD using SD. I also run a Time Machine backup to a different EHD. In Time Machine preferences, I have the SD EHD excluded. The problem is, sometimes TM forgets to exclude the SD drive and copies that as well; it then won't make additional TM backups because there's not enough space. Is there anything I can do to prevent this from happening? Is intermittently forgetting to exclude a specified drive a known TM problem?

[dnanian]

Not one I'm aware of. That seems weird... perhaps it occurs when you erase the drive?

[Estuary]

The TM EHD has been in use for a few months. TM erases older backups to make room for newer ones, so it erased all its prior backups to make room for the one that included the SD backup. I found a posting on the net recommending removing the SD backup from the TM exclusions list and then reentering it there. I did that today, and TM now seems to be working as expected. But backups are supposed to occur dependably, and it obviously doesn't serve TM's purpose if it erases all prior backups because it forgets to exclude the SD backup. I'm now wondering if using TM is more trouble than its worth, and to simply rely on SD backups.

[dnanian]

It's weird, because my Time Machine seems to have no problem ignoring external SD destinations...?