Copying Photos to External Drive

Hello--
I have a Hazel rule that I'm using the make backups of my Photos.app library and for the most part it's working great. The rule monitors the "Masters" folder within the Photos library package and when new photos are added copies them to an external drive and sorts them into subfolders based on date. I also had the rule add a comment to the original file so that it wouldn't continue to process the same files--which may not actually be necessary--I'm not sure.
This was all working great until I had my external hard drive unplugged for about a week and added several photos to my library. When I returned home and connected the hard drive Hazel didn't take any action on the newly added photos (but still processes photos I've added since that time).
My suspicion is that the rule was triggering while the external drive was unplugged and labeling the file it was preparing to copy. Then the drive wasn't connected to the copy obviously was never made. Then when the drive is connected again Hazel doesn't operate on the file because of the condition I set to not operated on files with the comment added to keep it from reprocessing the entire library (again this may not be necessary--but I was afraid since I was only copying files and not moving Hazel would see every file as meeting te condition again and try to copy them).
So my question is primarily--is there a way to only have the Hazel rule run when my external drive is connected? If there isn't a way how can I make sure that Hazel doesn't reprocess my entire library every time, but still ensure new photos are copied when the drive is reconnected.
Thanks so much for the help!
-Chris
I have a Hazel rule that I'm using the make backups of my Photos.app library and for the most part it's working great. The rule monitors the "Masters" folder within the Photos library package and when new photos are added copies them to an external drive and sorts them into subfolders based on date. I also had the rule add a comment to the original file so that it wouldn't continue to process the same files--which may not actually be necessary--I'm not sure.
This was all working great until I had my external hard drive unplugged for about a week and added several photos to my library. When I returned home and connected the hard drive Hazel didn't take any action on the newly added photos (but still processes photos I've added since that time).
My suspicion is that the rule was triggering while the external drive was unplugged and labeling the file it was preparing to copy. Then the drive wasn't connected to the copy obviously was never made. Then when the drive is connected again Hazel doesn't operate on the file because of the condition I set to not operated on files with the comment added to keep it from reprocessing the entire library (again this may not be necessary--but I was afraid since I was only copying files and not moving Hazel would see every file as meeting te condition again and try to copy them).
So my question is primarily--is there a way to only have the Hazel rule run when my external drive is connected? If there isn't a way how can I make sure that Hazel doesn't reprocess my entire library every time, but still ensure new photos are copied when the drive is reconnected.
Thanks so much for the help!
-Chris