Time machine can't select drive (but the computer see it)

Hi guys, I have a 1 TB WD elements external HD with two partition, one used for archive (200gb called "HD") and the other partition used for backups (either for macbook pro and iMac, 800gb called "Backup"). The last week I replaced my MacBook Pro retina mid 2014 15" with the "new" 2015 retina 15" version (the one without Touch Bar), that is running high Sierra 10.13.1. When I try to configure time machine on this computer I can't select the "backup" partition as backup disk! What could be the problem? With the iMac (running el capitan) everything works fine, booth the partition are HFS and the strange thing is that the MacBook clearly see the two partition and it can read it! I also try to check the drive with disk utility and it's ok. Any suggestion? Thanks