You can run Bitcoin Core prune node, not Bitcoin Core full node.

With prune node, you can choose how many latest GB of the blockchain you want to store on your disk. 15GB to 30GB is good for a prune node.

Could your internal drive store the extra 450GB?

If it can then you could make a drive partition for it and reinstall your operating system just on main partitions it already used (make sure you actually concentrate when setting things up though as you might overwrite it) - also I don't know if windows or mac like to reinstall over the whole drive so you might have to look up how to make a host protected area (HPA) that they then can't touch.

There's the alternative chance that if you have a few GB free you could make the new partition and slowly move things across and then resize the partition.

You can copy part of the blockchain, until your external drive is full. The rest of it has to be downloaded again I'm afraid.

As far as I understand, he's reinstalling an OS? Thus, he needs to move the blockchain data externally, wipe out everything internally, re-install the system, and move the blockchain back into it. If he can't move part of the data outside the internal drive, it needs to be re-downloaded.

For me is better to look for a hard drive that can accommodate all the data to avoid mistakes you might regret later. Speaking out of experience

google:
"The rev*. dat files (the "undo files"), contain all UTXOs spent by the inputs of a block. It was introduced in Bitcoin Core 0.8, and contains a concatenation of records, one for each block. This mimicks the structure of the blk*"

it basically holds a copy of txs that are spent tx....(opposite of a utxo set) incase there was a re-org/rollback the node doesnt have to go around requesting old blocks that got previously spent to be relisted as unspent, to then update the utxo database

its more of a internal time saver in cases of rollback

and yes many advise its helpful to copy the rev files too

(my personal node had full blockchain but i edited it to only have a certain amount of other files like rev files as its not often needed to re-org that far back, but still helpful to have recent rev files to speed up any re-org)

i have not personally messed to much with my version. so best to try to copy as much as possible to save as much time as possible
rev files should be ~60gb total

if you are only copying the first 50% of blk's then just copy the reciprocal first 50% of revs so they can correlate to the blk data

copying over just blk may save time of having to re download the initial block download process. but without the other files like rev, your system then need to read each block again the create the utxoset and rev sets. which is a bit extra of "rescan" time as they call it