Zenon Stoikos wrote:
I have been backing up for some years my iMac (1TB memory) onto an external drive (1TB).
The external drive now shows 323GB of 1TB available. Time Machine shows backups from 4 years past.
(a) Does this mean that I can still continue backing up ...
Yes. The subsequent clause "until the older backups come closer to recent" is not a factor. Older backups become deleted automatically as Time Machine replaces them with newer backups, when TM requires the space they occupy.
On that subject if you intend to use a significant percentage of that Mac's 1 TB startup drive, you will find a 1 TB backup drive to be insufficient. Apple has traditionally recommended using a backup drive of at least twice the Mac's internal capacity, and some references suggest 2x to 3x its capacity. As with most "rules of thumb" that one is overly broad and lacks strict justification, but if you follow that advice it will keep you out of trouble.
It's up to you to determine if you really need an item that was backed up four years ago, or ten, or twenty. Yes TM has been around nearly that long.
(b) Where in iMac can I see how much of memory has actually been used?
Memory is not a factor. Settings > Time Machine will show the backup date ranges.
(c) If it is time to replace the external drive, what happens with the current iMac memory? Do I delete it somehow? And retrieve from the external drive?
Memory is not a factor. Replace the backup drive when it no longer works, becomes unreliable, etc. For that reason it is best to have more than just one backup drive. TM will back up to two separate backup drives, or three, and so forth. Each drive will contain the bare minimum of one fully restorable backup.
