The original Apple IIc ROM Version 255 was able to boot ProDOS diskettes from an external 5.25" Drive by issuing the command PR#7. Unfortunately this feature was removed in later ROM versions. Still it is possible to boot from an external drive by typing the following code in the Monitor
300:A9 E0 A0 01 A2 60 4C 0B C6
This code was extracted from the 255 ROM listing and works with any later revision.
In order to boot a DOS 3.3 diskette using this method, the diskette has to be patched first using the Apple-//c-Boot Patch