I received an email today promoting a 200Gb SD card for £25 (7dayshop) and thought that this may be a simple to install solution for those running out of HD storage on their Mac.
You can boot of them too (at least up to High Sierra). It may be worth noting that he MBP’s use the native USB bus but the iMacs use the native PCI bus so the transfers may be quicker on the older iMacs if this is important.