[ubuntu] Trying to install ubuntu mini

Welcome. If the first six installs worked I'd stick with the ISO and the software you used used to make the USB for them rather than downloading a new ISO each time. If it worked once, the ISO is fine. There shouldn't be any need to wipe the drives everytime either. All you need is free space on a partition somewhere and choose 'Something Else' at the partitioning section of the install and install to the free space.

From what you're saying seems to be specific to grub installing. Could you install bootrepair and just run the bootinfo script (not any repair) and post the link it spits out here, please?

Have you tried another USB? Are you trying to install to the same place on the same drive each time? Are you trying to install the grub to sda and it's throwing that error?

As for the USB, you should wipe that and create one FAT32 on it prior to using UNetbootin. Not doing so can cause issues. I use Gparted (it's default in the live session) to rewrite the partition table on the USB which kills the partitions then I create one fat32 partition then UNetbootin it.