Where's all the fun in learning, hitting a wall ("WTF did my backups stopped working!?"), identifying a problem ("WTF is my USB drive full!?"), and trying to figure out a solution in all that ("Ohhh maybe I should read the instructions - hey look there, an exclusion file will solve my issue")!!
From the -OP-
NOTE: If you do go down the path of
backing your USB drive to your USB drive, it's possible,
not recommended. If you accept this risk, you want to also make sure you
exclude your backup folder name in the exclusion file, so you don't back up your backup folder, which will lead to exponential growth of your backup files. The safest way still is to store backups is far away from the device being backed up... so use at your own risk.
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When people run backups against their backups... they should also be seeing a tar warning message stating that "filename.tar.gz is already part of the backup and skipping it" or something like it... hopefully that would throw up some red flags.
But I would agree with you,
@ExtremeFiretop ... perhaps grepping to see if that path exists in the exclusion file, and if it doesn't, throw up a warning (along with the other adequate warnings on why it's a bad, bad idea to backup to your one and only local USB drive, and ask for input to see if they want to have backupmon add the exclusion in for them. Thanks for the idea!