Make a symlink in your SugarSync folder that links to a folder on your external drive:
ln -s /Volumes/ExternalDrive /Volumes/SugarSync/ExternalDrive
Note about hard links
Hard links do not normally point to directories, and they cannot link
paths on different volumes or file systems.
Note this risk:
BUT... what happens if the external drive/SD card is disconnected at
any point? Would those files be considered deleted, then propopagating
a deletion of files across all my computers??? I asked tech support,
and they said there WAS a risk of file loss. I did not test this
further. Anyone else?? I'm reluctant to test with my live data, and
have reached storage capacity."
It's likely that this risk will also apply to solutions that don't use symlinks. Unless the sync'ing application (sugarsync in this case) is programmed to be aware of such a location not always being available to the system.