I have to read many existing CSV files on a External Drive and combine the in Sequence (Sequencing is Critical) with restore point and write to output.csv on same External Drive in different path. Example A.CSV, B.CSV and so on to Output.csv , I am always appending to output.csv but there are high probability that IO operation fails. Like when writing B.CSV after A.CSV, if say B.CS has Character from A to Z, and IO exception happens after writing M, when I rerun the program , it should reprocess B.CSV and append O to Z to Output.csv. In my business case. output.csv going to be very big file in GBs though source file will be in 3-5 mbs max so do not want to reprocess it from start rather to restore writing where it fails. I am keeping the file names in Database Table and keeping updating the status as "Processing" and then Processed. Thanks and looking for your input.

using var fs = new FileStream(file, FileMode.Open, FileAccess.Read);
            using var reader = new StreamReader(fs, Encoding.Default);                
          (StreamWriter)  _filewriter.Write(Environment.NewLine + reader.ReadToEnd());