20+GB OST won't Defrag

A file that size is not going to defrag easily with the built-in or freebies (as you've already observed). As for why -- it's just too big, and the built-in as well as most freebies were not designed to handle single files of that size. Considering the size of that one file, the disk must be pretty large and to boot the disk needs at least 15% free space to defrag with the built-in and with most freebies.

Here's a white paper with some info for you as tech support: http://downloads.diskeeper.com/pdf/Is_Preventing_Fragme...

Your client can defrag large files and large disks (into multiterabyte sizes) easily with Diskeeper 2011. I suggest you have them get a free trial to defrag those large files and get their performance back on track. Since Diskeeper 2011 also prevents fragmentation, they will notice the difference in the performance and they may decide to keep it -- or not: it's up to them (either way, you solved their problem).

They can get free trials at http://www.diskeeper.com/defrag.

Bill R TechSpec