Liber wrote:It would mean to back up about 400Mb, format the drive to EXT4 and copy my data back. It sounds easy but it is quite time consuming operation with real throughput of the drive attched to Raspberry Pi approximately 2Mb/s... Is there any safe software solution for direct conversion NTFS to EXT4 ?
I don't think so, but even if there were a way you would want to make a backup first.
I would connect the drive to a Windows machine and copy off the files. You can format the drive as well on the Windows machine if you temporarily boot one of the Linux live images. You may be able to copy files back to the drive after format as well but haven't tried that.
Note that I am getting 8MB/s from my Win8.1 to a 2T ext3 formatted NAS drive attached to my RPi and 5MB/s reading from NAS to Win8.1.
(wired LAN connection)
Unless specified otherwise my response is based on the latest and fully updated RPiOS Buster w/ Desktop OS.