I assumed, maybe wrongly, that whatever system is started will allow your named Hard Disk Drive to mount.
(Depending on the errors and the system, a volume with errors might, might not mount read-only.)
What response, please, to the following command?
cd /Volumes/Hard\ Disk\ Drive
If not possible to change to that directory, then the volume has not mounted.
Your idea of taking the computer to an Apple Store is good.
As not everything is backed up, prefer to regain access to data.
I never visited an Apple Store but if they have a Lion system free for this purpose, ask whether they can attempt a repair of your volume in target disk mode. With the more modern Disk Utility the first few attempts might fail — with each attempt apparently no different from the last — but (in my experience) for some types of error, repeated attempts will eventually succeed.
With target disk mode in the mix — and with the holder of the Lion computer taking a Console.app view of
/private/var/log/fsck_hfs.log
— things might be expedited by timely repeated diconnections, reconnections to the target volume.
(Hint: never take that approach without assurance, from the tail of fsck_hfs.log, that the system has completed its automated attempt to repair the dirty filesystem. Keyword: timely.)
With or without repair, with the error as previously reported it should be possible — with a suitable peripheral system — to gain at least read-only access to your Hard Disk Drive for exceptional backup purposes.
Side note
At this time, beware of rushing to experiment with any of the .dmg -oriented startup hacks/workarounds that are known to the public but not recommended by Apple. Such things may be not appropriately tested in the fullest range of environments.
Hope that helps
Graham
