Programming the older radios that absolutely need DOS AND a true serial port can be a real pain in the ass. Although DosBox is wonderful and can work with the USB to serial adapters much of the time, sometimes you just can't go that route. After spending the better part of two days (!) going thru the motions using GRUB, PLOP, various bootable media, I thankfully ran across the following article which is working for me. Perhaps it will help you, too.
TweakhomePC : Install Dual-Boot of MS-DOS 7.10 (FAT32) on Windows 7 computer (Win7 installed first)
Many thanks to TweakhomePC for ending my misery!