1. A method that is executed by one or more processing modules of one or more computing devices, the method comprises: encoding data using a dispersed storage error coding function to produce a set of encoded data slices; identifying storage units for storage of the set of encoded data slices; determining that a storage unit of the storage units is unavailable, wherein the storage unit is targeted to store an encoded data slice of the set of encoded data slices; selecting a foster storage unit of the storage units for temporarily storing the encoded data slice; and when the storage unit is available, transferring the encoded data slice from the foster storage unit to the storage unit.