oebaca,
I had similar issues. A few days ago I bought a Nano clone off of eBay that was supposed to 8GB. Turns out that it kept corrupting my files and rebooting when it would get to certain files.
What I have learned is that the memory on the device was compressed to show 8GB capacity, but that its real capacity is only 1GB. The device was very unstable until I followed the instructions in a guide I found on this forum.
Here is the thread:
http://www.mympxplayer.org/viewtopic.php?t=2400
After I followed all the steps I now have the actual storage space to use for files. The device is no longer unstable. I don't have the advertised space I thought I was buying but at least the device is usable now.
Good luck to you. I hope the formatting guide helps your issue.