Some systems may not boot properly from pendrives and some pendrives does not really work in this (DOS) mode.
What you wrote about the CD ISO is completely normal.
The CD seems to have no files on it (even when mounted) because the boot image is stored on a different part of the CD (it is an image of an 1.44 floppy disk and the files on that are accessible only when you boot from that CD. Then they appear in drive A: just like if it would be a real floppy disk drive).
So you may need to burn the ISO directly to a CD (or CD-RW media) and then you should be able to boot from it. It contains FreeDOS and HDSentinel which should run in all cases.
It may happen that it does not detect some disks when special disk controller(s) used as it is compatible with the generic ATA, AHCI controllers and some (mostly integrated) RAID controllers only. But at least the title of the software and the copyright (and possibly the detected disk controllers) should appear.
If this happens, we can investigate the current system to check if it possible to improve the detection.
For this, a Report menu -> Send test report to developer option (in the Windows version) may help lots as it contains information about the disk controllers and the configuration of hard disks (eg. which disk is connected to which controller and port).
There can be different issues, mostly with the pendrive. If it's a larger capacity, it is possible the DOS system can't access if properly (simply can't read HDSDOS.EXE). This may depend on the actual system, motherboard, BIOS also. In this case the boot CD method may work better.