@bade302b the main consideration when leaving an external drive plugged in to any PC is whether the PC will try to boot from that drive when you start up?  If there is no boot data on the external drive, then it will be ignored, also the same if the PC is set to boot from the internal drive first.  Turning the PC on / off makes no difference - any open files will be written out if needed. 

 @bade302b my own main computer is a laptop and you could invest in a choice of either USB storage media, i.e. tiny USB device that plugs in and is almost invisible, or a small USB external drive that could tuck away on your trolley.  Both take their power from the USB port, so only one lead for the drive. 

 @bade302b if you only make random backups months apart then you are putting your data at risk!  I have regular scheduled backups that run several times a week to different storage destinations.  Unfortunately I see too many PCs where they have no backups at all with all the extra work needed to recover from any disk issue, bad update, virus etc!