My HTC One M7 displays a red triangle and keeps rebooting itself. Please help!?

[AC Question]

This morning my phone decided to apply some sort of AT&T update without my consent and since then it's only displayed a red triangle in between bouts of rebooting itself.

Things I've done to try and resolve the problem:
I've wiped the cache
Discovered that the problem is something about 'Mount SD card failed'
And that's pretty much it.

I'd prefer not to factory reset it unless absolutely necessary and I can't really take it into the AT&T store/send it to HTC because I'm outside the US. Help would be very appreciated.

A possible cause for this problem was that I was trying to turn the phone on during the update, so I held the power button.

 

[B. Diddy]

Welcome to Android Central! Do you mean that you tried to turn the phone off during the update by pressing and holding Power? Were you successful in shutting the phone down? It's not a good idea to force a power down during an update, since that can brick the phone.

 

[silverman25]

I made an account to followup. Yes that's exactly what I did. I wasn't thinking straight because my alarm didn't go off as intended thanks to the update and I was rather tired. I'm not certain if I did manage to turn it off or not.

 

[B. Diddy]

To force a power down, you usually have to hold the Power button for at least 10 seconds. Do you remember doing that?

 

[silverman25]

I think it may have been more, but yes.

 

[B. Diddy]

That may be a big problem, then. I'm not sure if a factory reset would even help, if the firmware got corrupted during the aborted installation. Part of this thread might help, or the links therein: [GUIDE] How to Return to 100% Stock | HTC One (M7) | XDA Forums

 

[silverman25]

Thanks. It doesn't look like I'll be able to save anything. I've called HTC support and spoken with their online support. It seems like my best bet is to manually reinstall the operating system, which will wipe everything anyway.