It's seems to me that it's an issue with your sd card (most likely), or your sd card reader (less likely, but still an option).So, after having initial problems with the "boot.dat?" showing up even with the micro-sd card inside, i have since successfully created nand backup and emunand and installed and played some games with no crashes. But i'm still experiencing some issues (except the micro-sd refusing to be seen sometimes):
1) sometimes when turning the console on after "true power-down" (holding the power button for 10-15 sec.) the led blinks blue and then goes out completely and nothing happens. After that, pressing the power button does nothing, only after another "true power-down" it starts up again.
2) even when the led turns green, sometimes it crashes after the sx logo comes up, just black screen and that's it, not responding to power-on, the only thing left to do is same as before, another "true power-down". It happens both when booting normally (it boots into emunand by default) and if i press vol+ during sx logo and then select "boot CFW" in the menu. The "genuine boot" seems to always work correclty, so it means it has problems booting CFW sometimes? Why could this be? Did anyone else have these problems?
Backup, format to fat32, try again. If the issue remains, try another card. Make sure you're using a quality, real, name brand card. SanDisk or Samsung is most preferred.
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