Testing usb_ba198af9 with a 16GB (INTENSO) brand USB thumb drive I can get a file listing of said drive in /q if it is plugged in during boot and without loading the mass storage driver. That's the built-in driver of TOS, I guess.
If I insert the drive after boot and then start loader.prg, it creates a new drive letter "y" but
I've got plenty partitions in that machine, the last regular drive letter being "W" or even "X".
Edit: Why W or X? 'Cause HDDRIVER may or may not assign a drive letter for the DVD drive. There are partitions C: through N: on the main IDE drive connected to the CTPCI IDE port, followed by partitions O: through R: on a 4GB SCSI drive, followed by S: on a ZIP 100, followed by T: through W: on a SCSI card reader, and then sometimes X: for the DVD (by HDDRIVER for use as DVD-RAM) and Z: for the DVD (assigned by Extendos).
When booting MagiC with a compatible thumb drive attached to USB, TOS first assigns letter Q: as that's the first free drive letter when HDDRIVER has used up all letters including P: (the limit for plain TOS without extensions). Then comes the second boot after the start of MagiC, where HDDRIVER re-scans all drives, assigns drive letters C: through W: and then the USB driver in TOS adds X: for the thumb drive.
Sadly, with MagiC, the partition on the thumb drive (type $0C, FAT32 Windows) cannot be read. HDDRUTIL sees the partition, but no drive and thus doesn't offer to format or partition it.
In case of MiNT, there's no re-scan and the thumb drive retains the letter Q:, but the following partitions are invisible.
What a mess
Schneider CPC464 (long retired), Atari Mega ST4 (retired), Falcon+CT2A, Falcon+CT63+CTPCI+Radeon 9250, Milan040+SCSI+Rage