While gparted and many modern PC/Mac/Linux tools can now resize partitions non-destructively, traditionally, and on the Amiga, resizing a partition is a destructive act. HDToolbox should have given you a warning when you resized it.
So, what options do you have? First, do not format, write, or do anything with the space formerly occupied by DH7. Your data is still there, but the system does not have a recognized partition surrounding it. There are some powerful data/partition recovery tools (DiskSalv), but I think they were all written in the <4GB era. Using them on your drive now could make the problem worse. There
may be a newer (>4GB-friendly) partition-finder tool, but I'm not certain.
OS4's PartitionWizard could help if you have access to an OS4 machine.
In the absence of OS4, you can recover DH7 by restoring it to its
exact previous size - down to the cylinder. This will take some trial and error: resize, save changes to drive, reboot; resize, save changes to drive, reboot; etc. It could be a lengthy process - the precision of the size indicator in HDToolbox could mean that there are a dozen or more end cylinders for the partition that correspond to, say, "500MB". It gets even harder if you're in gigabyte territory.
EDIT: Ah, here we go:
This tool looks like it will do the hard work of finding the partition dimensions for you. I'm sure Thomas will chime in with some additional advice as well.
Also note that 3.9's HDToolbox lets you save a copy of your RDB out as a raw file or as a mountlist. It's definitely a good practice to make these sorts of backups with some regularity (and store them on another drive) in case of corruption in the RDB or accidental errors. That way you can easily restore your drive's dimensions.
