When git annex does fsck on (for example) a GPG-encrypted special directory remote, it first transfers the whole file into .git/annex/tmp directory. If your annex is on an SSD, it's a good idea to make .git/annex/tmp a symlink to say /var/tmp so SSD isn't worn down. This actually may be a better default.
If .git/annex/tmp is a symlink to another fs, then adding doesn't work:
It looks like it would be good to have two types of tmp directories here, one for adding, another one for verifying (and that one could be redirected off SSD).