That error is coming from client-src/sendbackup.c:
 821         access_result = access(device, amode);
 822         access_type = "access";
 823
 824         if(access_result == -1) {
 825             err = vstralloc("could not ", access_type, " ", qdevice,
 826                         " (", qdisk, "): ", strerror(errno), NULL);
 827         }

where 'amode' is R_OK.  You can simulate this with:
perl -MPOSIX -e 'print "yes\n" if POSIX::access("/", &POSIX::R_OK)'

Dustin

-- 
Storage Software Engineer
http://www.zmanda.com