Finally, I've tried with another sdcard.
Thanks to Estel, I can confirm that the issue is related to the sdcard.
In fact I don't have any problem with the other one (trascend class2 4gb).
There are on internet many post about issues with "card error 110 response 0x0 status 0x0". Generally they seems related to sdcard module error with newest sdhc.
I've found here also a solution, but I don't know how apply it:
https://bugs.launchpad.net/ubuntu/+s...ux/+bug/861668

P.S. I've also noticed that the problem is not related to the limit of 80MB, but it's more frequent when the n900 goes in stand-by or wakes up from it.