Why does Lightroom say I don't have enough storage for application on iMac?

If Lightroom is like any of the other Adobe products, i.e. Photoshop or Photoshop Elements, you should be able to set the disk where the scratch files are located:

So check Lightroom's settings.

Also, is 180 Mb all you have free on your boot drive? Or should that be 180 GB? If Mb then I don't know how you're even able to boot up. That's entirely too little of free space to operate with. It's generally recommended to have a minimum of 100 GB of free space to optimize system and application performance.

I believe the alert you are seeing is that the memory has run out, not your storage space. You will need storage space on your internal HD to manage Virtual Memory and that HD is needed for swap files to better manage the memory available. It does not matter that Lightroom was launched on an external HD.

For why you are running out of memory, it may either be:

• Not enough storage space on internal HD to manage virtual memory.
• Third party processes launching on start up consuming your memory. These do not appear in the alert when you have run out of memory, but will consume resources and conflict with the OS. The most common culprits are Cleaners (such as CleanMyMac), AntiVirus, or VPN software. Uninstall those and try again.