Supposedly "not enough space to install High Sierra"

[CLOD-HOPPER]

Hi, all: On my old iMac (mid-2010), there are about four partitions, all with an old operating system on them. For a long time, I have been running High Sierra as my regular OS, because that is the most recent OS that my hard drive will accept. On one of the other partitions, Sierra (not High Sierra) was installed years ago, and today, it occurred to me to upgrade it to High Sierra, thus giving me a fall-back system to use if my regular H. Sierra ever got into difficulties. Also, I wanted to experiment, as I had never upgraded any operating system before — I had always wiped the partitions clean, and done a brand-new installation. This time, I was trying to install Hgh Sierra, but "on top of" Sierra, as it were.

So, I plugged in the installer (housed on a flash drive), booted up with it, and commanded "Install High Sierra", and it seemed to go to work on the installation. About ten minutes later, an alert showed, which said, "You do not have enough space to install High Sierra". This came as a great surprise, because Sierra (the OS being upgraded) is on a partition that is over 150 GB in size. (Only 50 GB are in use, according to Disk Utility).

My question is, why should such an alert show itself, when, obviously, there is plenty of space to install High Sierra? And what can be done about it, if it will not install?

With thanks in advance, for any helpful comments,
C.H.

 

[MacBiter]

When you're in Sierra, how much space does it say you've got in its main drive? I assume you're using the Disk Utility in the main High Sierra but what does DU in Sierra say?.