Well that looks like a nice tool. but the problem isn't recovering DH7 so much as it is making some new partitions on the drive. it just doesn't recognize them. I make new partitions and they never show up so I can format them.
Make shore the automount option is enabled in hdtoolbox,
If its on, then maybe its because AmigaOS3.x does not really support large disks, latter some wrote a patch called TD64 to enable 64bit io support, then in 1998 H&P made NSD64 patch for AmigaOS3.5/3.9.
Some filesystems support TD64, and some support NSD64, if you use correct patch, with correct filesystem it should work.
NSD64 became a integrated part of AmigaOS4, and there for no patching is needed anymore, on AmigaOS4.
And then you have PFS but I think its limited to SCSI.
You can try to remount the volumes, after the patch TD64/NSD64 patch is loaded.
If you put this in s:user-startup, replace X with the real unit number of your scsi.device and DHX with real name of your partition.
sys:system/mounter device scsi.device unit X partions DHX
