Decipher TextMessage saves your iPhone text message and iMessage history in your computer user account's area designated for "app storage" as is recommended for third party apps on macOS and Windows. The great thing about this is that your DTM data is stored in your user account data (protected by your user accounts setup if you share your computer). The not great thing about user account storage is that it's usually on the "primary" hard drive, which can cause you to run into drive-space limitations on the primary drive.
Follow these steps to store your Decipher TextMessage data on an external drive
In Decipher TextMessage, click
Preferences. Towards the bottom, select
Move Decipher Media Data Folder...
Select where you would like the Decipher Media folder to be moved to.
Decipher TextMessage will take care of the process of copying the files, setting up the settings to know to look in the new location, and then delete the originals from their previous home. It may take a while if there are a lot of files, so please be patient! :)
When it finishes with that, you will see a popup message that the move is complete and that DTM needs to close. Click ok and it should shut down the app automatically.
Run DTM again, and it will now be running with its data folder at the new location. You can confirm this by clicking
Current Decipher Media Folder...
The backups made within Decipher TextMessage are also stored in this Decipher Media data folder, so backups made within DTM will be stored on your external as well after changing the Decipher Media data folder location.