I try and avoid anything over 49C with my mechanical drives. My Seagates and WDs in the Mythbox and backup array rarely get over about 45C. Normally they are between 34C and 40C.

During a RAID build/rebuild (or an initial mass data copy of any kind), the drive electronics are much busier than in steady state operation, so the drives tend to heat up a lot more.

In the case of the Seagate with shingled magnetic recording (SMR), it will be even busier (and thus generate more heat) than other drives because of the need to rewrite adjacent tracks while writing others.

Once you get to steady state operation, I would expect things to cool down considerably.

EDIT: Meanwhile, if you have a room fan of some kind, direct it at the air intakes NOW, just to keep things happier.

Cheers