Bummer! Why, if I had a dollar for every "microtrack f'd me" post, I could buy a 722! It's unfortunate that there don't seem to be many digi options.
I would guess that the internal battery in the microtrack has worn out and needs to be replaced. I think if you send it to them and they replaced it, it would be okay again (as good as an MT can be!) for a while.
Maybe it could get to the point of having an internal short and be a hazzard? Anyone remember the story of Spoogles? Store and charge that thing in a metal can or somewhere similarly safe!
The problem, I think, is that the internal battery is frequently going into charge mode and it is consuming so much of your portable USB power that there isn't enough left to power the mt. The wall power supply has enough extra capacity to power the mt and drive the charge circuit.
Another work-around might be removing the worn out battery. I found this page with some info on how to do that. Getting the mt open isn't easy... The advantage of this approach is you don't need to deal with future battery replacement or explosion.
http://www.nauta-rcs.it/Instruments/RASP/RASP_timer/NAUTA_RASPtimer_prepare_UK.htmThe downside is we don't know if the mt firmware will allow the unit to operate without a battery present. If that is an issue, it might be possible to trick it with an appropriately sized resistor. There is a charging circuit there, and we really don't know what it will do..
If the USB power was sent into the battery leads, it might trick the charging circuit to stay off all the time.