I have an old backup external HDD. I used it to back up an old Mac. I still have the disk but I no longer have a Mac. Is there any way to be able to get this data from the HDD?
If you just want to explore and retrieve files you don't necessarily have to install a file system driver for HFS.
There is also HFS Explorer which will let you do that. (Java required)
There are HFS+ drivers available from Paragon that you can use to mount and access Mac drives. You can also find info and downloads here with driver files that were extracted from Bootcamp (so they are actual drivers from Apple) that can be installed to achieve the same thing. I bleieve all of these solutions are read-only, but it would work for retrieving your files. If you used some kind of proprietary software to do the backups, you will also need to find software that understands the file format used, but if you simply were copying files to the backup drive, simply using these drivers should work.