How can I transfer all my photos from my iMac to a disk

Two ways:

1 - copy the iPhoto library package to a disk formatted OS X Extended (journaled) with ownership set to be ignored. This will preserve all of your original photos as well as the edited versions and all of your organizational efforts you've put into the library.

2 - select all of the photos in the library and use the File ➙ Export ➙ File Export with Kind = Original and Subfolder Format - Event Name.

Select the destination to your external hard drive and you'll get a folder for each of the events in your library containing the original image files in that event.

How can I transfer all my photos from my iMac to a disk

How are your photos stored on your iMac? In an iPhoto Library A Photos Library? In your Pictures folder?

If it is an iPhoto Library, do as OT described in the screenshot. If it as a Photos Library it will depend on whether you are using iCloud Photo Library or not. Moving a Photos library that is syncing with iCloud requires additional steps.

If you imported your photos to your iPhoto Library or Photos library with the Preference "Importing > Copy items into the Library" disabled, use Old Toad's method 2 to export the photos to a folder. In that case copying the Library will not copy the original image files, since they are not included in the library.