That (freezing at 99%) happened to me when I was updating from Beta 4 to Beta 6.
I simply waited to make sure the green LED in the RaspberryPi is not flashing. Then turned off power to the ASIAIR. After that, Force Quit the ASIAIR app (on an iPad, double click on home button, and flick the ASIAIR off from that window).
As a side note: the red LED on a Raspberry Pi indicates power to the unit is on. The green LED on a Raspberry Pi indicates the processor is accessing the SD card -- never, ever, ever turn power off when you see the green LED flashing. Just wait a second or two from the last time you see the green flash before turning off power, and you should be safe.
Wait a short time (10 seconds or so) after removing power before reapplying power to the ASIAIR again. In my case, I also removed all USB connections from the ASIAIR, but I don't think it is necessary.
Then just power the ASIAIR back on, wait until all the red/green LED flashes during booting have quietened down, then reconnect the tablet to the ASIAIR WiFi access point, and relaunch the ASIAIR app. On mine, the ASIAIR app connected to the ASIAIR device right way, and showed the correct updated firmware version (top right hand corner of initial window).
Further, since you have previously backed up your SD card, there should be zero concern, since at the worst, you will just need to restore the card from your backup.
As long as the green LED is not flashing, you will not harm the ASIAIR or the SD card by turning power off.
Regards,
Chen
