Is the USB disk set with permissions to allow FMS12 to have read and write permissions?
Caveat: I do not have experience with setting 12 to use a removable drive path yet, so cannot say if this is possible. Just know the permissions would have be correct before it might work.
I don't think I would attempt that. Knowing that FMS pauses the files at the tail end of a regular backup, doing so while backing up to a USB drive is going to impact users more than it needs to. Why not use OS-level scripting to take a recent FMS backup and zip it and push it to the USB drive? Much more efficient...
I think somewhere in the docs is stated that FMS 12 backups to local NON REMOVABLE volumes only.
Anyway, the best solution or work around for you is to make the weekly copy to the USB drive via a command procedure an schedule that procedure.
Open a texteditor (like Wordpad) and write the commands:
COPY D:\<Directory and File Path to your backup> <USB Drive>:\<Path to destination directory>
Save this file as TEXT ONLY an put it to ..Filemaker Server\Data\Scripts\ directory. Naming it 'MyWeeklyScript.cmd'.
Now in the FileMaker Admin Console create a new weekly schedule and choose 'run a system script' as the action.
Choose your just created 'MyWeeklyScript.cmd'.
Make settings for the time and frequency to run the script and save.
Test your script by manually running the schedule and make sure that the schedule reports the current date and time for a successful run AND that you really have a copy of your data on the USB drive.
Thanks for all the advice. The backup was done late at night so would not have impacted users. I've taken the recommendation by Volker as the easiest for me.