An image is a good strategy to back up a working system (OS and installed programs), but a bad strategy to back up data. You are committing everything to a proprietary format in a huge file, both of which introduce a significant possibility of data loss. There are plenty of backup programs that will copy your data files in native format (windows readable), that will put only the individual file at risk, rather than the whole bundle. With the very large hard drives available now, the need to compress backups is largely history. A huge drive, like a huge file, can represent a lot of data loss if it fails. So long as it is just a backup (i. e. there exists another secure copy of the file somewhere else) that is not a major problem. But if you are using the drive for primary, albeit off the system, storage, then a couple of smaller drives may make more sense. The disk imaging system introduced in Win7 is still available in Win10 if you want to use that, although Microsoft seems to be less enthusiastic about it these days.
http://www.pcworld.com/article/3011736/windows/how-to-create-an-image-backup-in-windows-10-and-restore-it-if-need-be.html Also note that NAS drives are specifically designed to work in an NAS environment, where it is assumed that they are part of a fault tolerant array. In practical terms that means that they will retry reads on questionable sectors fewer times than a desktop drive, on the assumption that the same data can be recovered more rapidly by failing the drive and getting it from another mirrored drive or using a parity drive. A drive designed for desktop use is less likely to result in data loss if used as an individual outside a RAID system.