Expanding a dispersed storage network (dsn)

A computing device of a dispersed storage network (DSN) comprises:

• an interface;
• memory;
• and
• a processing module operably coupled to the interface and the memory, wherein the processing module is operable to:
• when a read expansion condition is met for a set of encoded data slices that is stored in a set of storage units of a first DSN, wherein the set of encoded data slices includes a pillar width number of encoded data slices, a read threshold number of encoded data slices, and a decode threshold number of encoded data slices, wherein the decode threshold is less than the pillar width number and is the minimum number of encoded data slices needed to recover a data segment:
• determine whether a difference between the pillar width number and the read threshold number is greater than or equal to the read threshold number:
• when the difference is less than the read threshold number:
• retrieve, via the interface, an expansion number of encoded data slices of the set of encoded data slices from a first subset of storage units in the set of storage units, wherein the expansion number is equal to or greater than the read threshold number minus the difference, and wherein the expansion number is less than the read threshold number;
• send, via the interface, the expansion number of encoded data slices to storage units of a second DSN for storage therein;
• receive, via the interface, a plurality of read requests for the set of encoded data slices:
• send, via the interface, a first group of the plurality of read requests to a read threshold number of storage units of the set of storage units of the first DSN for processing, wherein the read threshold number of storage units include the first subset of storage units;
• and
• send, via the interface, a second group of the plurality of read requests to a second subset of storage units of the set of storage units of the first DSN and to the storage units of the second DSN for processing, wherein a number of storage units in the second subset and the storage units is equal to or greater than the read threshold number.