"Why not do some benchmarking first to see if the disks really are the problem"

Yes, well, the disk queue length is regularly in the hundreds, which is why I suspected...

NULLs are yummy:

CREATE TABLE dbo.marketing

(

AccountManager varchar(30) NULL,

...

Are you planning on using the built in Windows defragmenter?  It may or may not handle your large DB files.  It wouldn't defrag the ones here and we had to...