I created a bootable USB drive and it works fine, but I want to utilize some of the free space on it.

How do I mount it as read/write?


Partition Type: Hidden HPFS/NTFS (Bootable)

Device: /dev/sdb1

output from mount command:
/dev/sdb1 on /media/master/Ubuntu-MATE 18.04.1 LTS i386 type iso9660 (ro,nosuid,nodev,relatime,nojoliet,check=s,map=n,blocksize=2048,uid=1000,gid=1000,dmode=500,fmode=40 0,uhelper=udisks2)

Your output shows you have an Ubuntu iso file and that it is as are most all, an iso9660 filesystem. An iso9660 filesystem is read-only by design and can't be changed by standard un-complicated methods. How does the ntfs partition fit in? Do you have the iso on an ntfs partition? If the device in question (sdb1) is the flash drive in reference, you can use another Linux OS and plug the flash drive in, make sure sdb1 is not mounted and shrink it to just over the size of the iso and then create another partition or partitions on it on which to store data. To access data on this partition from the Ubuntu on the usb, you will need to create a mount point and manually mount it each time you boot. You might be able to shrink the partition and create another from windows Disk Management if that's what you are using.

Sorry...

I just had to create a new partition.

Mental error! Not the first, won't be the last.

Thanks for the help!