What Naz to choose?

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For your needs of file storage and backup across various file types like images, CAD, videos, and office files, with a preference for RAID 5 or 6 redundancy and quick performance over a 1Gb network connection, the Synology DS1821+ would be a suitable choice. With its eight bays, it can accommodate your mix of 12TB SATA drives, providing ample storage capacity of up to 48TB. This NAS offers robust performance and reliability, ensuring smooth operations for your small office network. While you're accustomed to using QNAP servers, the DS1821+ from Synology is a robust option that fits within your budget of £1500, excluding hard drives. As for FreeNAS, while it's a viable option, the DS1821+ provides an integrated solution with user-friendly software and excellent support.
https://nascompares.com/nas-builder/