The drive is probably formatted FAT32 since Macs cannot modify NTFS files.
Vista should still be able to handle the files since Vista has no problem
with FAT32. However I suspect that the drive was formatted with a Mac and
Vista may not be able to mount it. Do you have another computer which you
can hook it up to and see if you can pull any files you need off of it?
What you need to do is get the files off, hook it back up to the Vista
machine, use Drive Management to delete the partition, recreate the
partition, and assign a drive letter. You shouldn't have any issue with it
after that.

"tgeorge" <tgeorge@discussions.microsoft.com> wrote in message
news:9F8D2C76-4E7F-4885-9C52-817DEA0C5100@microsoft.com...
> Many thanks, Colin. I had not found the Disk Management function
> previously.
>
> Unfortunately, the problem persists. The disk appears in Disk Management,
> its status is Online. However, when I right click the volume, the option
> to
> Change Drive Letter and Paths is grayed out. The only options are Delete
> Volume and Help (which doesn't have an answer for this issue).
>
> Here's a piece of information that could be the root of the issue: the
> data
> on the disk was written from a Mac. But it was written to be readable by
> both Macs and PCs.
>
> Any thoughts? Again, many thanks.
>
> "Colin Barnhorst" wrote:
>
>> Is it assigned a drive letter in Drive Management?
>>
>> Right click Computer, choose Manage, expand Storage, and choose Disk
>> Management.
>>
>> "tgeorge" <tgeorge@discussions.microsoft.com> wrote in message
>> news:A5B9369B-14C0-486A-9EAB-86401B9D9D4E@microsoft.com...
>> >I am connecting an external hard drive (Acomdata 250G drive - pretty
>> > standard) via USB to my HP laptop running Vista. I am able to see the
>> > device
>> > listed under Disk Drives in the Device Manager. Drilling into
>> > Properties,
>> > I
>> > see that the device is running properly and, having re-loaded the
>> > device
>> > drivers, I have the most recent drivers.
>> >
>> > The issue: I am unable to see the device listed in Window Explorer, and
>> > thus
>> > I can't drill into the folders to get at the content.
>> >
>> > Any help is greatly appreciated.
>>