Hi,
Several years ago, I did a successful HDD restore on my midway "nba on nbc".
It has similar hardware as your gauntlet legend.
Before : Faulty endless boot & reboot:

After : restore of hdd form mame chd (on same original hdd, by the way)

All the story :
http://www.gamoover.net/Forums/index.php?topic=8265.msg89061#msg89061to explain the whole process, I did a small tutorial on Gamoover.net, french arcade board.
http://www.gamoover.net/tuto/restaurer-un-hdd-sous-xp-avec-chdmanEven if it's in french, there are many screenshots that could help you.
At that time, if i remember well, no regular software was able to detect the original midway format/partition.
...no regular software but chdman.
Even if you want to backup you original, I could tell you that it isn't necessary, mame chd will work like a charm
the day you'll need it.
What is your exact problem using chdman ? Any message ? Maybe Win7 is responsible in a way or another.