ReadyNAS Duo and odd drives

The only problem you may run into is if one of the drive has slightly less space on the drive. If you establish the array with the larger one and attempt to add the smaller, it will be rejected. You will get a message in Frontview (the message will be logged) that the drive you are attempting to add is too small. It only takes one less sector and unless you do extensive testing, you won't know which one it is. It can even happen with two drives of the identical model. I have 10 ST31000528AS drives that were acquired some 18 months ago (8 in original service and 2 spares). While the service has changed, I had two of the drives in my old NV+, and recently got a message that a drive had failed in the array. The first two spares I installed were rejected as too small. I then had to open a refurbished drive received in replacement of a failed drive. It was accepted.

It is always recommended that you backup your data in the situation of replacing a drive. When the resync process takes place the drives run actively for a period of hours and the stress can cause the failure of a second drive if it is weak. Even in an array of three or more drives, if in a single redundant situation the data could be lost. In the case of a two drive array it most certainly would. Besides, maintaining a current and complete backup is always a good idea, as it is never a good idea to trust your critical and important data to only one device, be it a single drive or a single multi-drive device.