how do I Back-up the mini AND the USB drive to the TC ?
We will assume that the backup drive has been formatted for Mac and Time Machine
Open System Preferences (the gear icon on the dock)
Open Time Machine
Click Options
The next window will display a list of devices that have been Excluded from Time Machine backups
By default, the USB drive will appear here if it is formatted correctly for Mac
Click on the USB drive name to highlight it
Click the - (minus) button at the bottom of the list to remove the USB from being excluded from Time Machine backups
Click Save
Close the windows
Now, since you have removed the USB drive from being excluded in backups, the USB drive will be backed up along with your Mac
I do not know if , cannot tel if the data on the USB drive is also going to the TC
Currently, it is not.....unless you have already previously performed the steps listed above. If you have, then the USB drive will not appear in the list of Excluded items from backups.