I don't know what do you mean by USB mounting from DOS?
With AROS One x86 I tested on "ACER ASpire 5920" 2 external USB HDs, one with a FAT32 partition and the other HD with 2 NTFS Partitions, both HDs were mounted automatically by "Trident", see the two screenshots !
Found only "a small problem" on the volume name when from the menu you run "disk information", it was an old problem that seems to have returned, I will make the report to deadwood.
For the installation of AROS One I recommend that you create a Pendrive, that way everything will be fast (Install AROS One in just a few minutes).
To create the Live PenDrive follow the instructions found on the DVD