Hi
I've run a few NAS on my network over time but am far from experienced. Currently I have a DLINK ShareCentre 2gb (Raid 1) setup. I use the NAS as a backup only, not seeking to access files to work on in real time. With all NASs on my home network, cant say I've been impressed to date (regretted each purchase).
The key issue I have is the NAS seems to 'drop off' the network when transferring large numbers of files when connected wirelessly (while my network maintained full signal strength etc). Hence when doing a copy-paste of my images (around 80gb) for a backup the files wont all copy. On the occasion I have resorted to using a 32gb memory card to transfer the files. [once the files are on the system, tools like SyncToy seem to run ok as they only need to move small amounts of data]
Another issue IMO is that it appears you can't encrypt the data on a NAS (as per my internet searching and reading of the TruCrypt manual) nor can you install a HDD password. You can still however encrypt data that flows wirelessly via your router setup. Naturally this is only an issue if you are concerned that somebody will break in and steal your data. For me, this means I don't use a NAS to back up my tax records or other data that can be used for identity theft.
A third issue that may apply to you (it certainly does me), I enjoy USB v3 file transfer which is very quick and now am dissapointed when waiting on the comparitevly slow speed of the NAS to transfer data. For info, my network is IEEE a/b/g/n so is the current standard. Perhaps I compare the technologies unfairly.
As you can tell, I haven't got an IT background but hope my experience may assist. For the future, ill look to run with the external HDD.
Cheers
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