Continuous backup fails with no message
True Image Home 2014 on Windows 7 64-bit, with all updates from Microsoft. HP laptop 5 years old with 4 GB RAM and half-full 500GB hard drive. Backup is from C to a 1 TB portable USB2 drive F, with about 70 GB free space. Trend antivirus active.
Both weekly scheduled incremental backups and continuous backup were working fine until 2 weeks ago. Then Acronis started acting strange. Got messages that it could not back up to removable drive G (I have no idea what drive that would be; if I plug a USB stick into the other USB port, it is assigned H). Discovered that somehow a second incremental backup had appeared in the list, with G as destination. Deleted it and incremental backups were fine again but the continuous backup appears not to have run since the trouble started.
The status bar on the left of its entry in the backup list is green (the similar bar for the working incremental backup is purple) and the button with a square Stop icon is selected. When I click on the triangular Go button a progress bar appears with the message that it is calculating a time estimate. It grinds away for several minutes showing no progress, then the progress bar vanishes and the Stop button is again selected. No error message at all. That could indicate that it is active and working, with no new files to save, except that its status seems to indicate that it has not backed anything up for weeks.
Restarting the computer (required for several recent Microsoft updates) did not help.
Probably there is a log somewhere that will indicate what is going wrong, but I don't know how to access it.
All suggestions welcome. I'm a retired programmer, so I'm not afraid to use console tools if they are necessary.