Often cannot access music library

I have an issue accessing my music library that use to rare happen but is now increasing in frequency. I'm not sure if it's related to LMS or something else on my computer.

Here's my setup:

LMS 7.7.3 loaded on a Windows laptop running Vista SP2 (and Norton Antivirus if this matters)
Portable USB HD that holds my music collection connected to laptop
Logitech Touch
IPeng HD 1.4.16 loaded on an iPad mini

The problem that's occurring more frequently is when I bring up iPeng, it connects to LMS (I can access my squeezebox.com) but I cannot see or access my personal music library stored on the USB HD mentioned above. Also, LMS will not scan for music files. I noticed when this happens the hard drive light (the internal HD, not the portable USB HD that holds my music collection) on my computer is continuously lit up like it's working on something. This can go on for many hours before the internal HD light goes out and LMS returns to operating normally.

Any idea what may be causing this? Any settings I should check? Norton reports no viruses or malware, but could it be something is taking over my computer at times?

I'm a computer novice, so I really don't understand what's going on, but I'd like to resolve this so I can enjoy my music collection whenever I want, not just when the computer/LMS decides to let me!