You can try also this. Not sure whether it helps, but trying it won't harm anything:
My Garmin forerunner was locked on the 'transferring data' screen and this worked for me without losing any data:
For the 910XT you might need to try a master reset. It's a little tricky and requires some dexterity. Good instructional video here:
Glad you got it working. Apologies to Garmin for delaying a new device purchase.
You saved me -- Thank you! Soft reset worked!
My Garmin forerunner was locked on the 'transferring data' screen and this worked for me without losing any data:
Thankyou! This just worked for me also. My 910XT was frozen while in the middle of transferring data.
OMG, you're a lifesaver or should I say a Garmin saver...lol. I just did my best 3k since my knee replacement surgery and Garmin locked in transferring data screen. I did the steps you suggested and it worked without losing my data. Yipee, now I can go brag to my friends...lol