mongorestore wants the specific collection’s filename rather than the entire database’s dump folder:

dwiel@dwiel$ mongodump -d db -c variables -o ../backups/1
connected to: 127.0.0.1
DATABASE: db to ../backups/1/db
db.variables to ../backups/1/db/variables.bson
3 objects
dwiel@dwiel$ mongorestore -d db -c variables –drop ../backups/1
ERROR: root directory must be a dump of a single collection
when specifying a collection name with –collection
usage: ……..
dwiel@dwiel$ mongorestore -d db -c variables –drop ../backups/1/db/variables.bson
connected to: 127.0.0.1
../backups/1/db/variables.bson
going into namespace [db.variables]
dropping
3 objects