Quick tip for an error that seems to plague a lot of Pro Tools users. I recently installed Pro Tools 8.0.5 on Windows 7 Home Premium 64-bit and was constantly getting the dreaded "H/W Buffer Underrun" error (-6085). I only ever record a voiceover track in Pro Tools (occasionally with a music bed in another track), but even with a small session such as that it would happen consistently, sometimes after only a few seconds, but at best I'd get 5 minutes of recording time before it would throw the error. It would also throw the error when bouncing a mix to MP3, and the length of the bounce didn't seem to matter much.
To try and fix this I tried (almost) everything listed in Avid's Windows 7 Optimization Guide to no avail, and also scoured the far reaches of the interwebs looking for a solution with no luck, so I finally ponied up the $39 for a support ticket and gave them a call. (As an aside, Avid's support is top-notch. I was on hold for a while but once it was my turn I got exceptionally good assistance.)
For reference the machine I'm using is a Lenovo G770, which has an Intel Core i5 (2.4 GHz), 8GB of RAM, and a 5400 RPM (yes, slow) drive. I also have a 1TB USB 3.0 USB-powered Western Digital hard drive attached to this machine, but regardless of if I recorded to the main drive (the one with Windows and Pro Tools on it) or the external drive the error would occur. At first I was a bit worried about the hard drive speed but Avid said it should be OK, but they did recommend that even if you use a USB drive it's better to record to an external drive. The other tidbit the support tech shared is that solid-state drives aren't yet recommended for Pro Tools recording.
After the support tech ran me through most of the things I'd already tried, had me delete some preferences files, and then gave me some other settings to try, I was initially able to get a 10 minute stretch recorded without any problems so I optimistically tried to record some stuff for real, but after only about two and a half minutes the error occurred again.
As what I thought was a last ditch effort I tried the one thing I hadn't yet tried since it seemed like a stretch: I disabled both my wired and wireless network cards. That, in combination with reducing the number of CPUs from 4 to 2 and setting the utilization to 90%, seemed to do the trick. Ultimately it may have only been the networking cards that were causing the problem but I violated a major rule of troubleshooting and changed two things at once, and now that it seems to be working I'll probably just stick with these settings.
If you search for the -6085 error in your favorite search engine you'll get tons of suggestions and unfortunately you may just have to experiment, but in my case once I disabled the network cards I recorded for 20 minutes without a hiccup which was quadruple what I'd been able to do before.
The Avid support technician did tell me that in Pro Tools 10 they leverage RAM more in the playback engine instead of streaming to disk so heavily, so if all else fails you may want to upgrade. I personally didn't want to do that because I really don't need anything that Pro Tools LE 8.0.5 doesn't do so kudos to Avid for still supporting it, and supporting it so well.