Will Time Machine delete Drobo 5 oldest back up once red warning light?

I have a Drobo 5 to protect iMac for six years. At the beginning stage, I inserted one 1T, two 2T and one 3T, it works fine these years. Every time once yellow warning lighted, I replaced a bigger capacity disk to replace the small ones, then it return to green and works smoothly. After six years, my Drobo 5 has one 3T, three 4T plus a 6T, much much bigger than before. This time, yellow warning lights again which indicates the 3T disk needs to be replaced by bigger disk. I suddenly thought that my iMac only has one 3T hard disk, why I use total 21T to protect a 3T disk? After check the back up history, the oldest back up was happened July 2015, a lot of back up in Drobo 5. I know that Time Machine could deletes the oldest back up when disk becomes full, so, why I need to buy more and more bigger capacity disks to replace it every time? This has become an endless game, as no matter how big the total capacity I increase, soon or later, it will be full if the oldest backup are still kept in it.
My queries are,

  • May I leave the yellow warning and do nothing, and let Time Machine deletes the oldest back up automatically?
  • Or, just simply delete the oldest back up until the Yellow or even Red back to Green?
    Any suggestions is appreciated, thanks.

This is what happens when you use a Drobo as a TM backup target without enabling the backup volume & setting a size for it.

You’ll probably need to use TimeMachine to delete your existing backups & start over to fix it.

Go here Drobo Online User Guide

in the left pane.
Expand your model
Expand “Setting up your Drobo …”
Select the entry "Using your Drobo 5xx With Time Machine
Follow the instructions.

When TimeMachine is using that volume it will indeed delete old backups when the space you set for it to be able to use is filled. If you just use the standard Drobo volume TM will just keep filling it & the unit will keep asking for bigger drives till you hit the 64TB volume size limit (then TM start deleting ancient ones), before that it will keep filling the Drobo till it runs out of actual space, since it looks to TM like there’s more space than there actually is…

I’d suggest setting the Backup volume to either 2 or 3 times the size of the drive you’re backing up.

Many thanks for your prompt guidance. I will do and let you know the result, again, thanks.

It took more than nine hours to delete all 2015 backups, and more than eight hours completes deleting all 2016 back ups. Very slow, looks like I better to reset Drobo 5D and start over to fix it, or I probably have to spend another four days to delete all back ups.

The speed of those types of operations is one of the penalties for how TM keeps the space requirements down. If you carry on then at least theoretically deleting backups should get faster as there become fewer of them, again that’s a function of how TM stores stuff to avoid storing duplicates.

If you’ve space to store any other data you might have on there while you do it, then a reset is probably faster.

I just removed Drobo from TM and use a 4T external disk for TM, it is backing up now, and says need anouther 3 hours. I will reset Drobo, once it done. After reset, will set up a volume for TM by dashboard, thanks.

After volume set up, the first back up took about 11 hours, very slow, but success. Now everything looks fine. Thanks for your help!!!