The problems are random, unpredictable, and varied, BECAUSE it is being used in a way other than it was intended. Potential problems: Data corruption. This is the number one reason not to do it. The array is susceptible to data loss due to unprotected read/write errors during the rebuild. Doing this frequently further increases the chances of data loss. If you are moving 2TB of data, and swapping them daily, averages say you will experience some data loss every 6 weeks. A "real" backup solution has the ability to error check and verify inconsistencies in the data. If a single disk in a degraded array is the only source of the data, there can be no verification or correction - it is simply lost. Unresolved Foreign Configurations. At some point, your disk will show up as "foreign". In a perfect world, all you need to do is clear it and assign it as a hot-spare to rebuild. The server assigns its configuration at boot to determine which configurations are foreign to it. If the controller "forgets" its configuration and has to decide which disk is the "good" one, it may choose the wrong one, or it may try to mesh them together (data loss). Occasionally, these foreign/local configurations get lost and can't be resolved.
I am using a PE T610 w/Raid 1 pair as a backup destination. The pair having three members. One of the members to be swapped out alternately daily to the data safe. When the drive from the safe is pulled out and replugged into the hot bay it should rebuild with the spinning drive right? Giving me a pair spinning and yesterdays member in the safe? Old members should also rebuild automatically right?  Comments please...
Hot Swap Raid striping 1 with 2 cold members