Hey, not sure if you fixed it by now, but I just ran into the same issue - “Unable to Extend” error present only with ‘generate direct’, but not ‘generate buffer’ on an external drive using (I assume) Linux. What file system format are you using for /mnt_ext/seagate3? I checked the code and direct mode uses fallocate to initiate the plot on Linux, but fallocate is only supported for ext4/xfs filesystems per the man page.
I had direct fail on an exFAT-formatted external drive that worked perfectly once I reformatted to ext4. Could be helpful in the code to have a check for the target filesystem not being one of the above before using an alternate method to carve out space (ex. dd if=/dev/zero …).