Currently, NewEgg has a sale on this
WD Blue 4TB Desktop Hard Disk Drive - 5400 RPM SATA 6Gb/s 256MB Cache 3.5 Inch - WD40EZAZ US $84.99 + Free S/H - US $5 (promo code: EMCEXET26) = US $79.99.
TBH, I don't know how good and/or reliable the above HDD. However, I presume it is a low-powered HDD with 5,400 RPM and it should be good for an old Seagate GoFLEX/Home NAS.