datadir – New download of bitcoin core v28.0, how to use blocks which have already been downloaded to my machine?

How can I get [the current version] to use the blockchain data which is already on my machine from [prior versions]

I believe the appropriate process is something like this:

Close bitcoin-qt, bitcoind etc. Make backups. Check backups.
Locate the Blockchain data.
E.g sudo find / -name blk00000.dat
Configure Bitcoin Core to use that location. vi $HOME/.bitcoin/bitcoin.conf if directory isn’t in default location where bitcoin looks. See datadir and walletdir
If needed, move a copy of important wallet.dat to appropriate location. See existing guidance.

For comparison, here is the response for getblockchaininfo on a Bitcoin node that hasn’t been run for 6 months on a PC that has inadequate resources (100% disk usage)

{
  "chain": "main",
  "blocks": 849345,
  "headers": 869149,
  "bestblockhash": "0000000000000000000090dd75955aeb49fb6a51f259f3d6ee3c9639ce241ced",
  "difficulty": 83675262295059.91,
  "time": 1719257861,
  "mediantime": 1719254246,
  "verificationprogress": 0.9679465468853898,
  "initialblockdownload": true,
  "chainwork": "0000000000000000000000000000000000000000803db1ef35254b7ce5b55f8a",
  "size_on_disk": 660000239919,
  "pruned": false,
  "warnings": ""
}

I think a key thing here is the number of blocks the program knows of.

This PC has a blocks folder with about 615 GB of data in it and a chainstate folder with under 12 GB of data in it.

Your equivalent information does support your conclusion that the program isn’t seeing all your data. Maybe it has logged a reason why it isn’t using all the data (e.g. some error message concerning a block file?)

In the file debug.log I see

2024-11-05T18:22:16Z Bitcoin Core version v24.0.1 (release build)
2024-11-05T18:22:16Z InitParameterInteraction: parameter interaction: -listen=0 -> setting -upnp=0
2024-11-05T18:22:16Z InitParameterInteraction: parameter interaction: -listen=0 -> setting -natpmp=0
2024-11-05T18:22:16Z InitParameterInteraction: parameter interaction: -listen=0 -> setting -discover=0
2024-11-05T18:22:16Z InitParameterInteraction: parameter interaction: -listen=0 -> setting -listenonion=0
2024-11-05T18:22:16Z InitParameterInteraction: parameter interaction: -listen=0 -> setting -i2pacceptincoming=0
...
2024-11-05T18:22:25Z Default data directory XXXXXXX/Bitcoin
2024-11-05T18:22:25Z Using data directory XXXXXXX/Bitcoin
2024-11-05T18:22:25Z Config file: XXXXXXX/Bitcoin\bitcoin.conf
...
2024-11-05T18:22:47Z LoadBlockIndexDB: last block file = 4309
2024-11-05T18:22:48Z LoadBlockIndexDB: last block file info: CBlockFileInfo(blocks=84, size=126890608, heights=845620...845702, time=2024-05-29...2024-05-29)
2024-11-05T18:22:48Z Checking all blk files are present...
...

Note the last few lines above. You’ll have to search for the latest occurrence of these to check that the program is actually looking in the place you instructed.

Your debug.log should show you where Bitcoin core thinks your data is located and this should help you work out what is happening.

There may be other error messages earlier in the debug.log that explain why the program needed to do what it is doing.