Steam is downloading games again from 0

[ExpertMedal7]

I started the download of cod and went to sleep.When I woke up today,I noticed that steam said download error,not enough disk space.I checked my drive and it had 45GBs left.I thought that it might’ve been a glitch and checked the downloaded files folder of cod.It has 75GB worth of downloaded files on it and it’s the exact size which the game has.I thought it might’ve been a glitch and copied the files from downloading to the common folder and restarted the download so steam can verify those files and use them instead of redownloading.Steam begin verifying files and after verifying copied over the files to downloading automatically but not all of them.Steam began downloading again from 17GB and the rest 44GB was left untouched by steam in the common folder.It’s redownloading the files even though it has already downloaded them.How do I fix this issue?

[Ettanin]

Steam needs up to three times the installation size in space:
Installation pre patch
Installation post patch
Patch itself

It keeps a backup of game files pre-patch while patching, to be able to roll back the update in case of patching failure.

It keeps a backup of game files post patch while the update is incomplete to continue anytime where it was interrupted.

Those backups are deleted when updating was done successfully.

[ExpertMedal7]

Ahh but 40GB was already downloaded last night.Now it says 16 out 75 instead of 40 out of 75 yesterday