I'm trying to apply encryption at rest to a system of 6 files using the developer utilities. The process works as expected for 5 of the files, but the 6th file—which has c. 26GB of container data in open external storage—consistently fails the encryption process. The log file reports
7/4/2018 12:58:02 PM Due to an error encountered, File encryption/decryption was skipped. File:EMR_Data.fmp12  7/4/2018 12:58:02 PM Destination File could not be created, and all further processing on it was skipped. File:EMR_Data
A file EMR_Data.fmp12 of the expected size is in fact created in the expected directory, but its external storage directories are not created.
I thought I'd try encrypting again with the container data held inside the file, but alas I'm also unable to change the container field's storage setting:
Can any one suggest what is needed to allow encryption to be applied to this file with external container storage?