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 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 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.