Annexed data is stored inside your git repository's .git/annex directory. Some special remotes can store annexed data elsewhere.

It's important that data not get lost by an ill-considered git annex drop command. So, git-annex can be configured to try to keep N copies of a file's content available across all repositories. (Although untrusted repositories don't count toward this total.)

By default, N is 1; it is configured by running git annex numcopies N. This default can be overridden on a per-file-type basis by the annex.numcopies setting in .gitattributes files. The --numcopies switch allows temporarily using a different value.

git annex drop attempts to check with other git remotes, to check that N copies of the file exist. If enough repositories cannot be verified to have it, it will retain the file content to avoid data loss. Note that trusted repositories are not explicitly checked.

For example, consider three repositories: Server, Laptop, and USB. Both Server and USB have a copy of a file, and N=1. If on Laptop, you git annex get $file, this will transfer it from either Server or USB (depending on which is available), and there are now 3 copies of the file.

Suppose you want to free up space on Laptop again, and you git annex drop the file there. If USB is connected, or Server can be contacted, git-annex can check that it still has a copy of the file, and the content is removed from Laptop. But if USB is currently disconnected, and Server also cannot be contacted, it can't verify that it is safe to drop the file, and will refuse to do so.

With N=2, in order to drop the file content from Laptop, it would need access to both USB and Server.

For more complicated requirements about which repositories contain which content, see required content.

See also walthrough/backups for some details about copies.
Comment by obergix Sun Aug 25 05:37:04 2013