netinstall (6.14) with routeros-x86-6.14.npk (and the NTP, KVM, user-manager if you use one of that) and select "keep old configuration" before reinstall
"keep old configuration" never worked for me saing "can not mount partition", I've reinstalled by booting from CD.
Actually this is not so fun if you upgrade router in the remote location.
This is very bad bug, confirming that guys from mikrotik have a little testing before release.
I am already was afraid of remote uprading, and with this problem this is more sad.