Time Machine backup of an external disk....?

Thanks for your answer John! The external disk essentially contains my photo-library being 600Gb of folders with Raw/CR2 files in it. Also some libraries with MP4 video. The iMac internal disk contains Adobe Lightroom catalogs and other work-files, miscellaneous folders with normal stuff being documents, email, iTunes music and films. All pretty important to have a decent backup! :-)

Thanks for the info John, Old Toad!

It seems that as long as you make just one TM backup everything is under control: one full backup followed by hourly incremental backups of both disks(internal, external). But if I add a second backup disk to make a second backup, TM seems to not being able to cope with that. It starts doing multiple full backups of unchanged data of the external disk, which is not workable. So I am back to just one single TM-backup. I will have to do a second backup using other tools.

I organised meanwhile around a second backup with CCC; works perfect indeed! I also like the incremantel backup from CCC which seems to update the actual bootable backup image directly, it seems.

One question though... I also tried to boot from the bootable CCC backup, using a USB connected WD external disk, but that seems as good as unworkable because of the slow response of the USB disk. I suppose you need a faster connection to use it as a real boot backup. Would a Thunderbolt 2 or 3 connected drive solve that problem?? Any experiences?

Wisk you a strong 2018!!🙂