Because of the "usb-modules first (so that memory-stick becomes /dev/sda1)"-behavior (see line 234 in linuxrc in miniroot.gz) there are problems while booting from an usb-device (which does not work because the basemod will not be found):
1.) insmod usb_storage is not working, because scsi_mod is not loaded.
"unknown symbol scsi_report_device_reset" and so on ... and then
can not insert /modules/usb/usb-storage.ko unknown symbol in module (-1): no such file or directory
That happens because the needed /modules/scsi/scsi_mod.ko will be loaded after that.
So to solve the problem, i added to my linuxrc an insmod scsi_mod before the line that loads the module usb-storage. But i could not test, if this is causing problems with real scsi devices (i have no such devices) later, when all the other scsi modules are loaded.
2.) there is no sd_mod in the miniroot and it is not loaded before insmod usb-storage
If sd_mod is not there the usb-device (maybe scsi- or sata-disks too) will not be accessable as /dev/sdX and will not be found and mounted when searching the iso.
So i copied sd_mod.ko from the base-iso into my miniroot and like in point 1 my linuxrc loads it before loading usb-storage.
Please add sd_mod.ko to the standard-miniroot - module-directory, too.