Hello there. Yes, that is what I do. I keep my hard drive with me with photos and the LR catalogue *.lcat on the hd, thanks to which all the changes are always up to date wherever I decide to see the pictures. A tip might be to set up your LR to save catalogue each time you exit the programme so you can start where you finished even if that's on a different device (look for "catalog settings"). Hope this helps!
Well, the workflow is fine to use the external drive and keeping your images and Lightroom catalog file on it, however as you have the second system iMac is a little issue because when you connect the drive to windows system it will assign a drive letter to it but in iMac it will read the hard drive by the manufacture name.
therefor you files would be missing every time to connect the hard drive on iMac in Lightroom catalog.
Check the Discussion : Lightroom Catalogue external drive to be accessed by Mac and PC
Let us know if that helps.
Very helpful, thanks. Link explained it well.