The following worked for me but can't guarantee it'll work for you.
I had the yellow triangle with Lag fix, I eventually plucked up the courage to remove the battery. Sat there for 5 miutes wondering whether a) I'd brick my phone and b) If not whether I'd wipe everything off of it.
In the end, neither or at least for me. The reason I had the yellow triangle was because I booted up holding the Vol Down, Home and Power button all at the same time, when it's meant to be the Volume Up button.
As I say, please use at your own risk. I was surprised that the Downloading message doesn't time out with no source available for the downloading. Perhaps someone will come on here and detail a less harsh way of getting out of the yellow digging triangle screen involving holding down combination buttons?