That is expected, because the posters are not stored in the database files. You would have needed to backup the Plex data folder almost in its entirety. (with some exceptions) This script applies these exceptions automatically: PSA: Script to back up and restore Plex app data on a Windows system
If you don’t want to use that script, mount the old hard drive and use e.g. robocopy to copy the whole plex data folder from the old to the new server.
If you also need to preserve all the home page customizations and the server shares to other plex users, you better also copy the registry data: https://support.plex.tv/articles/201539237-backing-up-plex-media-server-data/
If the source drive has been deleted/formatted already, you have no choice but to Refresh Metadata on each of the libraries – and hope that the automatically fetched posters are to your taste.
If you have manually picked different posters before, these choices are now unfortunately lost.
You also want to verify that all your media files are still accessible under the exact same address as before (drive letter, folder name, file name, and server name + share name for network file shares).
