We would be happy to sell the pile of these Seagate drives we have...
I would go for cheap and back up now that the Core has an easy way to back up and retrieve data.
IMO, go with a WD Red , an HTC or a Toshiba (they are well rated) and be ready if it fails.... These days, the failure rate is in practice not so different form brand to brand.
I use WD Red at home and in my tests/supports/shows NAS drives with no issues so far...
I wouldn't specifically recommend the use of "NAS" targeted drives such as the WD "Red" series drives in the Core - let me explain why...
A WD “Red” would work perfectly well in a Core however there are settings on the WD Red drives (as with any other "NAS" oriented drive) that makes them more suited to a multi-drive mirrored / RAIDed environment than for standalone use. One of these is a setting called “TLER” – Time Limited Error Recovery - which is the amount of time that the drive will reattempt to read a block of data before giving up and declaring it as “unreadable”. In a NAS targeted drive this value is set to a short time period so that the drive aborts reading a difficult block of data quickly (knowing that the block can be read from the mirror drive or recreated from the parity drive of an array) so that the NAS itself is able to get on with servicing other data requests and doesn't stall. If a NAS targeted hard disc drive (WD Red, Seagate IronWolf etc.) is used in a single drive environment then in the event of a bad data block being encountered it will simply give up attempting to re-read that block earlier than a “desktop” targeted drive (however that doesn’t mean that the desktop drive would be able to read the block – just that it would try for longer before giving up and saying “Nahhhh – can’t read that”).
It’s like comparing a coupe against the estate version of the same car – the estate will be better suited for some tasks than the coupe and vice versa but you could use either and, on the whole, be no noticeably worse off with one over the other. (Apologies to anyone offended by automotive analogies...)
Any 2.5” or 3.5” SATA hard or solid state drive should be absolutely fine in the core (we’ve not found any that don’t work) however our main testing has been done with the Seagate Pipeline / Video 2TB hard discs or Samsung 850 EVO 512GB or 1TB drives and these are the drives that we specifically recommend.