HDD drives such as this one tend to be cheaper than an SSD drive, but they're not as fast. This model has a 3.5" form factor, the most common for HDD drives.
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Synology HAT5310-18T - 3.5p - 18T - 256MB cache - 7200t/min - SATA 6Gb/s
SmartTeck
Synology HAT5310-18T internal hard drive 3.5" 18000 GB Serial ATA -
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