Here we would love provide some method, please try and feedback, we would try our best to help:
1. do overwrite formatting on your SSD and make sure it's exfat format;
2. check the disk partition table of your SSD, make sure there is no special setting and there is only one section;
3. plug the SSD after the Pro is completely rebooted and ready to work.
Also, please make sure you plug it via USB port, not the type-C port.