Mixed results there.

The good: Not only is the ability to dump regions confirmed, we now know that its nonvol memory is in region 154 and the path to card is 1. The earlier error from trying to dump region 41 via 1 is likely because that region doesn't even exist in the region layout for O750t.
The not so good: "Volume Not Mounted" error means that we cannot directly access the file system from the card and even with Mass Storage Mode enabled it likely wouldn't be accessible that way either without firstly some manipulation and a reformat.

The file system in most devices is actually contained in a dedicated region, either 48 mostly in older-style devices or 83 in newer devices and in some cases it's duplicated with copies in both regions. So we should try to dump both. You might need a minimum 8GB card for this attempt to work. Remove last_id.bin from 2512 folder and replace the contents of update.txt with the following:
Spoiler:
rrgn,48,1:/48.bin
rrgn,83,1:/83.bin
reboot