When several drives on different interfaces are connected, IDE drives have a higher priority for TOS than SCSI devices, and SCSI devices have a higher priority than ACSI devices. This means that if there are IDE and SCSI devices, drive C: is located on the first IDE device and not on the first SCSI device. If there are SCSI and USB devices, drive C: is usually located on the first SCSI device and not on a USB device.
There are scenarios where you may want to change the TOS device order. With HDDRIVER you can do that in HDDRUTIL's "Devices and Partitions" dialog. By dragging a device button to the location of another device, or by editing the bus/device ID after double-clicking the button, you can change the device order of HDDRIVER's device scan. This change does not influence the boot device order of TOS (first IDE, then SCSI, then ACSI), but it changes the order HDDRIVER uses for scanning devices and assigning drive IDs. If with a Falcon you place one of your SCSI drives before your IDE boot drive, for example, TOS will still boot from the IDE drive, but drive ID C: will be assigned to the first partition of your SCSI drive instead of the first partition of the IDE drive.
Changing the device order also works for USB devices, provided that the USB device drivers are launched as HDDRIVER modules. In this case you can tell HDDRIVER to scan for USB devices first, and for IDE, SCSI or ACSI devices later. This results in drives C:, D: etc. being located on your USB drive instead of your IDE/SCSI/ACSI drive, even though the boot drive used by TOS was your IDE/SCSI/ACSI drive. The resulting environment is the same as if you had booted from your USB drive, even though TOS does not know anything about USB.
Note that when re-configuring HDDRIVER in such a setup, the HDDRIVER.SYS to re-configure is not located on drive C: but on a different drive, usually the first drive on your actual boot device. In HDDRUTIL, use "Locate HDDRIVER" to tell HDDRUTIL which HDDRIVER program file to configure.
You can even disable the ID of your boot device in "Partitions and Devices", so that partitions on it are not assiged any ID at all. In this case TOS boots HDDRIVER from this device, but then "forgets" about it. This is an unusual setup, though, which is not recommended. You cannot easily re-configure HDDRIVER, because the partition with HDDRIVER.SYS on it is not accessible anymore. In such a setup re-configuring HDDRIVER may require booting from floppy disk first.