[cybmole]

at a guess, it is not actually a Calibre issue.

your previous PC probably had an adobe ID - that is the key needed for opening any DRM protected epub. you would have gotten a key ( just an email address known to adobe) at some point, and thus "activated" it as a one off operation in the distant past.

your new mac needs to be "activated" with the same adobe key. which should be quick & easy ( unless you need a lost password - in which case there will be a password reset service at adobe.com).

if you've forgotten that email address , then it gets harder, the answer is probably stashed somewhere in the windows registry, but hopefully it will not come to that. your epub e-reader may display it for you somewhere it in ts menu options.

on a new windows PC I would install Adobe Digital Editions and it would walk me through the activation process. I assume something similar happens with a MAC.

your reader devices will have gotten their adobe "key" from the PC when first connected so they should still be OK

NB simply buying a book and adding it to (vanilla) calibre does not change the book's DRM status in any way. so if they were readable on your old PC then you must have had a valid key.

if your issue is with amazon books, not with epub, then please clarify, that's an easy fix also.