When you have OtherOS.. you also have a standard EXT partition which can be read/mounted in your PC copy of Linux. Part of the check I think for OtherOS is that XMB has correctly created an OtherOS partition before allowing it to proceed. This why it may be difficult to get the original implementation running without re-creating this blank partition. Somebody with a pre-3.21 should see what a blank OtherOS partition looks like (formatted thru the old OtherOS menu options), just before Linux is actually copied across to it. This could help in replicating this on newer drives.
On non-OtherOS enabled machines, you just have your main/encrypted Sony-specific partition. This still can be dumped with Linux on your PC as a raw file with DD when you plug it into a sata port.