I think we can mount external storage and to set it as ancient data folder
--datadir.ancient value Data directory for ancient chain segments (default = inside chaindata)
Your problem is that you can start mining from whichever block and it's totally ok. At least in theory.
If we are now at block 10 and you start mining from block 5 you may eventually get your own block number 10 and 11. But at that point the rest of the blockchain is probably already at least at block 20 or 50 or 1000 - you can never catch up by solo-mining ...