Before dropping a file, git-annex wants to be able to look at other
remotes, and verify that they still have a file. After all, it could
have been dropped from them too. If the remotes are not mounted/available,
you'll see something like this.

	# git annex drop important_file other.iso
	drop important_file (unsafe)
	  Could only verify the existence of 0 out of 1 necessary copies
	  Unable to access these remotes: usbdrive
	  Try making some of these repositories available:
	   	58d84e8a-d9ae-11df-a1aa-ab9aa8c00826  -- portable USB drive
	   	ca20064c-dbb5-11df-b2fe-002170d25c55  -- backup SATA drive
	  (Use --force to override this check, or adjust annex.numcopies.)
	failed
	drop other.iso (unsafe)
	  Could only verify the existence of 0 out of 1 necessary copies
          No other repository is known to contain the file.
	  (Use --force to override this check, or adjust annex.numcopies.)
	failed

Here you might --force it to drop `important_file` if you [[trust]] your backup.
But `other.iso` looks to have never been copied to anywhere else, so if
it's something you want to hold onto, you'd need to transfer it to
some other repository before dropping it.