Can't add drive as Hot spare on TS 3400

[AugeK]

Hi.

I own a TS 3400 where I configured a RAID5 with 3 disks.
Now adding a 4th disk of same type and size, after formatting the size ist too small for Hot spare.
Without formatting the size is OK but disk is grayed out in RAID menu.
Disk ist GPT.

What did I miss?

[AugeK]

Problem solved by try and error: i selected the disk and then hot spare.
Just hot spare does the job