Read this page:
MS dynamic disk
and
here
I did a little reading for ya. Looks like you really really didn't want that dynamic disk and now you are in a pickle. I think dynamic drive allows it to be read by any OS over a network and is necessary for software RAID. DOS can't read from the dynamic disk and both DriveImage and Ghost must work under DOS. Partition Magic won't fix it.
You may want to reload XP on the new 100G drive. Then x-fer all your stuff from the stripe to the 100G. Then you can clear and reset your stripe, use DriveImage to x-fer the 100G to the stripe and use it from there keeping the 100G as your backup. Admittedly this is not a good suggestion but it may be your only option.
Hmmm, if it can be accessed over a network, maybe you could back it up to another computer over a network. Do you have another machine networked that you could image it to? Actually, this probably won't work. You can't image it if it is booted to Windows and if you boot to DOS with network support DOS won't see the drive. Argh.
Hopefully someone who knows more about dynamic disks will see this and offer a better solution. There has got to be a way to backup but it may be an enterprise solution that costs a lot of $$.
[edit]
Upon further searching I found this from Saltin:
For the record, if you converted the disc to dynamic under 2k, there is NO way to revert back short of formatting. Your data is gone if you revert, no two ways about it.