I am prepared to launch the personal-related (PersCatalog) but I'm not sure where it will load/save the Ircat file.
Simply load (open) a particular catalog. End of story. From an external drive, from an internal drive, from wherever you created it / wherever else you may move or copy it in the future; LR does not mind where. All LR minds about is whether the file and folder addressing is current and accurate within this catalog, for all the various photos which are imported to it.
Changes that you make to the photos within this catalog, immediately update the Catalog database (the thing which appears as a .lrcat file to the operating system, plus some related subfolders). There is no separate "save" operation needed, or possible.
Analogy: if you pick up a piece of clay and start sculpting it, there is no option to "save changes to the clay". OTOH there's no option to "exit without saving changes to the clay". Your sculpting has been directly CHANGING the clay itself, as you went along.
Hence you can just close the LR program at any point, or switch to a different Catalog, and when you come back everything will be still as you left it. If you ever want to discard some particular changes in LR, there are specific tools such as Undo, Reset, History, Snapshot. Or I suppose, if you wanted to throw away everything done within a Catalog since yesterday, you could restore a Catalog backup that was made yesterday and open that.
Also: There is no need to disconnect drives. LR will not get confused. There is no need for Catalogs to always be on the same drive as the photos they refer to. LR will not get confused. It's LR's job to manage and keep track of where stuff is; across multiple drives, if you like. Furthermore, if a LR Catalog is on a different drive than some imported photos, then even if that drive where the photos are stored is disconnected, the Catalog still shows thumbnails for these photos and still allows you to carry out organisational tasks on them.