Check whether the User you are migrating has profile with dedupe enabled option, if yes new storage location should be mapped with Dedupe Storage location. Then try for migration to new storage location.
Since you have upgraded from DLO 7.0 to 7.5 the profile that the users are using shouldn't have the "Enable Dedupe" option checked.
Check that and let the backup run as per schedule and see how it goes. It should backup the file on the dedupe storage and only the delta changes would be there on the rugular storage.