1. A method that is executed by one or more processing modules of one or more computing devices, the method comprises: encoding a data segment of data using a dispersed storage error coding function to produce a set of encoded data slices; identifying a set of storage units for storage of the set of encoded data slices; while writing the set of encoded data slices to the identified set of storage units: determining that a storage unit of the identified set of storage units is unavailable, wherein the storage unit is targeted to store an encoded data slice of the set of encoded data slices; determining to foster store the encoded data slice; when determined to foster store the encoded data slice: selecting a foster storage unit of the identified set of storage units for temporarily foster storing the encoded data slice; writing the encoded data slice to the foster storage unit, the writing including issuing a write request to the foster storage unit, wherein the write request includes the encoded data slice, an indication that the encoded data slice is to be stored as a foster encoded data slice, a second encoded data slice of the set of encoded data slices, and an indication that the second encoded data slice is to be permanently stored; and when the storage unit is available, transferring the encoded data slice from the foster storage unit to the storage unit.