Does deleting old Time Machine backups incriminate other backups?

You cannot delete backups with the Finder.

Each incremental Backup is based on the files stored in the previous Backup. If you do somehow manage to get them into the Trash, the Trash will not empty and they will be stuck there. if you do manage to get them deleted from the Trash, the Integrity of your entire set of backups will be lost.

You can use Time machine .app to request the deletion of ONE backup. Then you have to wait until it straightens out all the links before you request another deletion. The sizes of the backups NOT deleted will expand, because files from the deleted backups will have to be moved into a different backup or be lost.

If all this is based on the premise of copying an old Backup to another drive, that in itself is a Bad Idea®.

You should always make a new Backup directly onto the new destination drive, rather than risk replicating a yet-undetected error onto the new backup set.

Time machine will allow you to add additional drives. Each will be used as a stand-alone backup set, independent of all others. Every other scheduled Backup will be sent to every other drive. You may remove a drive at any time it is not actively backing up, and no damage will be done.

if you read a bit more carefully, Time machine deletes some daily and weekly backups along the way. Only when space is short does it delete OLDest backups. And Delete is probably not the right word for this process, because all unique files must be merged into another backup.

if you exclude system files (which are easy to recreate anew by a new Install), size of backup would drop, maybe by the 40-ish GB difference you are seeing.

Want some nice accessible articles on Time machine?

the late James pond wrote a wonderful set of very accessible illustrated articles, and they have been re-hosted by a forum senior contributor. Be sure to follow any (Next) links you see at the bottom of a page. Although these may seem dated, Time machine is nearly unchanged since these were written.

.Pondini's Apple OSX and Time Machine Tips

.

If you cannot stop yourself from revising your existing backups, I suggest you make a complete, exact copy of them onto the new, larger drive. Then check to be sure they are working on the new, larger drive by making a new incremental backup there.

Use only TimeMachine.app to delete old backups. You are limited to deleting one-at-a-time -- the old files need need to be consolidated before the newer deletions can be processed.

and all the old backupfiles from the 2TB backup drive will be downloaded onto the 6TB drive?

No, but sort of.

Copying backups makes no sense when the original files are close at hand, so Time Machine does not copy another backup set.

Instead, it will create a new, stand-alone backup set DIRECTLY from the original files, not from a backup set that (could possibly) have hidden flaws.

Every-other backup session will go to every-other drive for as long as both are present.

The new, stand-alone Backup set on the larger drive will have no files older than the day you start using it. So after a while of running both, you could set the older one aside as an Archive.