When adding evidence, you receive an error like "Add Evidence FAILED:..Unable to access Will attempt to rollback".
If the evidence cited in the error is supposed to remain in the case, you will need to locate that evidence and put it back in the expected location, accessible by the Processing Engine(s).
If the evidence cited in the error is no longer necessary, use FTK, connected to the Summation/eDiscovery database, to remove the evidence from the case.
The evidence cited in the error message is not accessible. Database Optimization (DBO) is run against all previously processed evidence in a case, so if evidence previously processed has moved, then DBO will not be able to complete.