I hear you on all the wall warts and AC plug-in devices! My study/radio shack (I'm an amateur radio operator - ham) has way too much plugged in already what with all my radios and computer equipment. I use the portable HDDs as backups and occasionally as temporary storage. My MBP only has a 256GB internal drive, and I find it starts to slow down if the available space on that drive falls below 100GB. So I keep most of my photography stuff on backup storage so I always have plenty of space available on the MBP's internal SSD.

I also find that the MBP will often slow down if I plug in one of the WD portable HDDs. My testing with the portable 256GB SSD plugged in does not show any slow down of the MBP, at least so far. So that is good. Plus the SSD is fast enough that I can place my photos for a collection on it, link to it via Lightroom CC 2015 or Capture One Pro 10, and do all the post processing without taking a speed hit. Basically I'm using the MBP for the processing of photos stored on the SanDisk portable SSD. If I did that with the WD portable HDDs, Lightroom would crawl along at a snails pace and Capture One Pro 10 would slow down quite a bit too. After I finish doing my post processing, I backup everything to other storage drives and unplug the SSD drive.