maybe you can use the (internal) cdrom/dvd connector with a adapter for a hdd?
also i would have said the same as abced but then i found this:
https://hedichaibi.com/fix-xpenology-problems-viewing-internal-hard-drives-as-esata-hard-drives/
that guys intension was to correct the numbering on his sata drives (a "gap" seems prevented the normal numbering in a row), but if you have a look to the last picture
he kind of accidently made his usb flash drive a intern hdd (disk21), so you might be able to convince dsm that a usb drive is internel (even if no one would recommend doing it)
i'm still not getting it why the flash drive is a hdd, from the configuration data it looks ok, only "mistake" i can spot is that he did not set the "maxdisks" drive numer (if not changed it will be 12 so the flash drive is number 13 and above the maxdisks)
@vladiwan
as you are new i dont think you should try this, if things go wrong or you make mistakes you can easyly mess up your system