If the capacity of the primary flash card is equal to or smaller than that of the redundant flash card, the router copies all the files from the primary flash card to the redundant flash card. However, if the capacity of the primary flash card exceeds that of the redundant flash card, the router creates a hidden synchronization reserve file on the primary flash card, provided enough space is available for the file.
The purpose of the synchronization file is to prevent the creation of data that cannot fit on the redundant flash card. The file contains no useful data, and does not appear when you view the files in NVS. The size of the file is equal to the difference in capacities of the two flash cards. For example, if the primary flash card has a capacity of 224 MB, and the redundant flash card has a capacity of 220 MB, the size of the synchronization file is 4 MB, and only 220 MB of space is available on the primary flash card.
If the primary flash card does not have enough space to create the synchronization reserve file, the synchronize command fails, and you see a warning message on the console. To resolve this issue, either delete unwanted files from the primary flash card or replace the redundant flash card with a higher-capacity flash card.