Hi @Mahesh Madhusanka ,
Welcome to Microsoft Q&A and thank you for your query.
Unfortunately appending to an existing blob using ADF copy activity is not supported. I would recommend you to please up-vote and/or comment on the below existing similar feature request from other users. Up-voting/commenting will help to increase the priority of the feature request.
Existing feature request: https://feedback.azure.com/forums/270578-data-factory/suggestions/36163726-append-replace-upsert-the-outgoing-data-from-copy
Additional info:
Please have a look at this SO thread and see if that helps as a workaround for your scenario: Is there any workaround for using Append blob in Azure Data Factory copy?
Hope this info helps.
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Thank you
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