Is it possible to send/remove files via USB, to write/read system partition from a computer, using WinSCP for example ?
Root explorer is fine, but i would prefer to do it from a PC...
Normally you cannot write to the system partition, as it is (and should be) mounted read-only, so before you
can write to the system partition you need to remount it read/write:
#adb shell mount -o rw,remount -t yaffs2 /dev/block/mtdblock3 /system
Then you can use adb to push and pull files to and from the device eg:
#adb push myapp.apk /system/app/
once you has finished writing to your system partition, you should remount it read-only for security reasons (protect your phone from bad behaving apps)
#adb shell mount -o ro,remount -t yaffs2 /dev/block/mtdblock3 /system
If you still get problems writing to the /system partition, you need to do it via recovery mode.
(boot into recovery, and mount /system manually from adb)