We had a customer that upgrade PGP WDE to 10.1.2(26). They where unable to mount the virtual disk they created on an earlier version of pgp. It gave them a generic unable to mound message. They took the .pgp files ( for the virtual disk) to a desktop that had 9.8.1 installed and it opened them fine. They then had to decrypt them, then upgrade to 10.1.2(26) and reencrypt them.
I did not find this documented anywhere. I would have called into support, but that has been very painful lately and slow. We were able to figure this out on our own and I thought I should post it as it may help others.
Note: we tried the tradition removing of the pref/policy .xml files, that did not help. downgrading to 9.12 allowed only one disk to be readable.