Hello!
Gerald_clark, it is FAT32 for sure (confirmed in other Linux machine and Mac)
Tinypoodle, the output is:
/dev/sda1 on /mnt/sda1 type ext4 (rw,relatime,barrier=1,data=ordered)
/dev/sda1 on /home type ext4 (rw,relatime,barrier=1,data=ordered)
/dev/sda1 on /opt type ext4 (rw,relatime,barrier=1,data=ordered)
/dev/sdb1 on /mnt/sdb1 type vfat (rw,relatime,fmask=0000,dmask=0000,allow_utime=0022,codepage=cp437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
AND... it worked! What I did was to delete all partitions, created a new one (FAT32) and it worked! I still don't know why it didn't let me write into it before but now it works!
Now I'm embarrassed about what happened and for not having thought of doing this before ... I'm sorry! And many thanks for the help!