Amazon has the Travelstar™ 5K1500, 1.5TB 9.5mm HardDrive on sale for about $120. I ordered one that just came in the mail today I am still cloning the drives (I had put the original 2 500GB drives in a hard drive enclosure when I installed an SSD a few months back). Tomorrow I will be cracking open the mini and installing this 1.5TB under the current 240 SSD. My thought is using the spinning hard drive more for internal file storage & the SSD for my system boot disk this drive will also most likely be upgraded this year to a 512 or 960 SSD then, I will move the 240 SSD to a 24″ iMac that I have that is currently running off of an external 500GB USB drive.
HGST Travelstar 5K1500 1.5TB 2.5-Inch Mobile 5400 RPM 9.5mm Internal Bare Hard Disk Drive 0J28001
Introducing the Travelstar™ 5K1500, a 5400 RPM, 500GB/platter, 2.5-inch hard drive that delivers a massive 1.5TB of storage capacity in an industry-standard 9.5mm design.
[alert ] *Update:[/alert]
This does require dismantling your Mac mini:
But, now I have a 240 SSD boot drive & 1.5TB of internal Storage
If you had a single drive mac mini you would need the data double kit (I was starting with a Mac Mini Server)