Bug BUG-000122188

Synopsis

Attempting to append data from an Excel of .csv file to an ArcGIS Online hosted feature service with a large number of fields returns the error message: "Error: Unable to Append Data."

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Additional Information

N/A

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Alternate Solution

Perform any of the workarounds below to resolve the issue:
Using ArcMap 

1. Bring the hosted feature layer into ArcMap and create a local copy. 
2. Append data into the local copy and sync it to the service. 

Using the Analysis Function in ArcGIS Online 

1. Publish the data as a hosted feature service in ArcGIS Online. This merges the data with the existing service.
2. Add both the services into a webmap and run the Merge Geoprocessing function (under Analysis in MapViewer) which outputs a new service. 

Using ArcGIS Pro 
Use the Append tool to load the existing data into the ArcGIS Online feature service. ArcGIS Pro provides the ability to edit feature services directly from ArcGIS Online. However, we have to note that the feature service does not have to be shared once logged into ArcGIS Online through ArcGIS Pro. Tokens are automatically appended to edits and etc. 

Reduce the number of fields in the original hosted feature service.