When you formatted that microSD card as 'Internal' it was a fundamental change to the card's functionality. Typically a SD card will be formatted as FAT32 or exFAT, the reasoning being most modern operating systems include at least basic (non-native OS's like Linux or Mac) to full (MS Windows) support for FAT file systems so the card is compatible no matter which platform -- i.e. you can transfer files from one computer to another that may or may not be running the same OS, or transfer files between a webcam and a PC, or whatever. But when you format the card as Internal, that changes the file system to ext4, the same file system as the internal storage media, and like the internal storage media the card also gets encrypted. So those two aspects essentially tie that card to that mobile device -- ext4 is a common file system in Linux based systems but out of the box neither MS Windows nor OS X support it (you need to install a third-party utility to add ext file system support), but that's basically a mute point as the encryption also prevents the card from being used elsewhere anyway. The encryption key specifically links the card to the device that encrypted it (the encryption key is stored on the device). So a microSD card formatted as Internal essentially becomes a fixed component. The Android OS on your device sees the card as just additional storage space, it's merged with the internal storage. You don't interact with the card any longer, the Android OS manages the card's content.
If later you opt to use the card in its original state, you can format it back to 'Portable' but do so following all the instructions that show up when you do. It's not a trivial process so. The card will be reformatted back to a FAT32 or exFAT, and the encryption removed but all data on the card is wiped in the process so expect to reinstall some apps if necessary, and saved data (backups are important).
As for backing up and restoring SMS messages, as others have mentioned try using SyncTech's SMS Backup & Restore app:
https://play.google.com/store/apps/details?id=com.riteshsahu.SMSBackupRestore&hl=en_US&gl=US
It's actively supported and developed, and I've found it useful for transferring text messages and phone call logs from phone to phone. If you do format your card back to Portable you use SMS Backup & Restore to backup to your card but a better, safer solution might be to use the app to backup to an online storage service. If you lose your Moto or it gets stolen, the backup goes with it if your backup is only on that card.
But it also depends on just which text messaging service you're using. If you're using WhatsApp for example, it's a heavily proprietary service so you're better off using the backup/restore function that's built into the WhatsApp app/service than a third-party solution.
