There are a few posts referencing SD card here. I'm not sure if some of the advice given is to use it to store routes on your XT, or not.
My advice is to use the SD card you place in the XT slot. Simply create a Garmin/GPX folder on the card and copy your GPX file directly into it. The XT will recognize it existence, import/recalculate it, and store it for execution. The advantage is that if your route ever gets corrupted, you can quickly re-import it from the SD card and be on your way. If you import into Garmin internal storage and corruption happens, you have to go back to MRA and perform the transfer process from scratch. Admittedly, this a rare occurrence, but the one time it happened to me was at the worst possible instance. As always, plan for the worst, but hope for the best.
In any case, a direct wired file copy (manually performed by you) from your PC/Mac to your Garmin is always the safest method. Anytime you use a transfer intermediary (Drive app, Tread app, MRA Connector), there is a possibility that the process can break. For reasons totally out of your control.