I bought a Dell Latitude E5420 some eighteen months ago, for exclusive Linux use. I've been mostly happy with it, although it's been liable to "run hot" when there's more than a couple of applications going (especially if one of them is either update-apt-xapi or Chrome with active Flash) and at that point the disk churns a lot, the fans come on, and the load average suddenly rockets despite there being very little CPU usage.
The iotop utility didn't reveal much: commands would sometimes thrash the disk a bit, but then only sporadically and for small volumes (especially compared to how much the disk icon was itself flashing.) I wish at the time that I'd known to make more use of vmstat, to measure how much the swap space was being thrashed, because I increased the memory from 1x2GB to 2x4GB...
... and the thrashing completely went away. It seems that my machine simply didn't have enough memory for what I wanted to run, and Ubuntu would have to keep writing not-currently-used memory pages to the swap space on disk, to make room for more data actually in memory. Admittedly it was rare that I would run the 1GB VM that would sometimes bring my 2GB host laptop to a standstill, but presumably other applications were just as memory-hungry..
There's no great moral here, and I appreciate I don't particularly come out of this well ("if in doubt, take the screws out" isn't the best route to debugging resource problems) but I would say that if your machine seems to be thrashing the disk for no good reason: try increasing the memory. But maybe check iotop and vmstat first, unlike me.
