You can probably do so with a little lateral thinking & manual intervention. You'd need to set up a Fast Backup profile with a dynamic Destination, as described in the contextual Help on the Fast Backup settings page (press F1 with page open / scroll to bottom and back up ~1 page-full). But, with the following differences (because you can't strictly control a rotating cycle (because you don't necessarily return every weekend), you can't use one)
Set the Full Backup path option to point to your ext HDD (optionally using a \SBFull\ folder or similar if you need to segregate the output from the profile from anything else on that disk)
Set the standard/main (i.e, if not a Rescan) Destination to point to your SD card and use the %DATE% variable in the string, e.g. X:\SBIncr\%DATE%\ (not %DAYOFWEEK%, etc - you can't use a cycling variable because you don't necessarily know when your 'cycle' will stop, and you may 'overshoot')
Set (leave) the Force Rescan When option as 'Never, Manually only'
Leave the 'Delete all the files...' unchecked (it generally won't work anyway, because your incremental backups will go to dated folders, so it will usually never point at the same incremental folder again). If you do use it, you are restricted to running your incremental backup just once a day (because doing it twice with this option set will erase any backups made by the first run that day)
Now run the profile with your ext HDD plugged in. The full backup will be created on it.
Put the ext HDD away and insert SD, and set off.
Next day (and every day thereafter till you return), run the profile. It will create Incremental backups in dated folders (as many as you need subject to length of trip and capacity of SD).
On your return: go into the profile, set Force Rescan (for next run).
Connect the ext HDD, remove the SD card, run the profile. This will update your full backup, and also reset/re-create the database ready for next trip
Repeat...
What you do about the old backups on the SD card/s is up to you and their capacity. One idea may be to have two SD cards, for last trip / next trip, and rotate them, manually erasing the old dated folders before re-use of that card. It shouldn't matter if any old Incremental backup folders are left on a card (because the above scheme won't ever reference them again once the date changes) - except for the space they take up...
Set the Full Backup path option to point to your ext HDD (optionally using a \SBFull\ folder or similar if you need to segregate the output from the profile from anything else on that disk)
Set the standard/main (i.e, if not a Rescan) Destination to point to your SD card and use the %DATE% variable in the string, e.g. X:\SBIncr\%DATE%\ (not %DAYOFWEEK%, etc - you can't use a cycling variable because you don't necessarily know when your 'cycle' will stop, and you may 'overshoot')
Set (leave) the Force Rescan When option as 'Never, Manually only'
Leave the 'Delete all the files...' unchecked (it generally won't work anyway, because your incremental backups will go to dated folders, so it will usually never point at the same incremental folder again). If you do use it, you are restricted to running your incremental backup just once a day (because doing it twice with this option set will erase any backups made by the first run that day)
Now run the profile with your ext HDD plugged in. The full backup will be created on it.
Put the ext HDD away and insert SD, and set off.
Next day (and every day thereafter till you return), run the profile. It will create Incremental backups in dated folders (as many as you need subject to length of trip and capacity of SD).
On your return: go into the profile, set Force Rescan (for next run).
Connect the ext HDD, remove the SD card, run the profile. This will update your full backup, and also reset/re-create the database ready for next trip
Repeat...
What you do about the old backups on the SD card/s is up to you and their capacity. One idea may be to have two SD cards, for last trip / next trip, and rotate them, manually erasing the old dated folders before re-use of that card. It shouldn't matter if any old Incremental backup folders are left on a card (because the above scheme won't ever reference them again once the date changes) - except for the space they take up...